MICROBIOLOGY AND MOLECULAR BIOLOGY REVIEWS, June 2001, p. 319–333 1092-2172/01/$04.00⫹0 DOI: 10.1128/MMBR.65.2.319–333.2001 Copyright © 2001, American Society for Microbiology. All Rights Reserved.

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Cytokinesis in Prokaryotes and Eukaryotes: Common Principles and Different Solutions NANNE NANNINGA* Swammerdam Institute for Life Sciences, BioCentrum Amsterdam, University of Amsterdam, 1090 GB Amsterdam, The Netherlands INTRODUCTION .......................................................................................................................................................319 CYTOKINESIS AT THE CELLULAR AND SUBCELLULAR LEVELS............................................................320 Organization of the Bipolar Spindle Apparatus ................................................................................................320 Location of the Division Plane in Eukaryotes ....................................................................................................320 Location of the Division Plane in Prokaryotes...................................................................................................321 Preliminary Conclusions........................................................................................................................................321 CYTOKINESIS AT THE MACROMOLECULAR LEVEL ...................................................................................321 Cytokinesis and the Cytoskeleton.........................................................................................................................321 Cytokinetic ring...................................................................................................................................................321 Actomyosin and associated proteins ................................................................................................................321 Microtubules and SPBs .....................................................................................................................................322 Microtubule-associated motor proteins ...........................................................................................................323 Small GTPases and the cellular envelope .......................................................................................................323 IQGAPs ................................................................................................................................................................324 Bacterial Cytokinesis..............................................................................................................................................324 Cytokinesis in E. coli ..........................................................................................................................................324 Bipolar organization of the bacterial replicating chromosome....................................................................325 Chromosome segregation...................................................................................................................................326 Does DNA condensation play a role during chromosome segregation? .....................................................326 How does E. coli know where to divide? ..........................................................................................................326 DIFFERENT SOLUTIONS .......................................................................................................................................327 What Is the E. coli Equivalent of the Bipolar Spindle Apparatus?.................................................................327 Exploring the Poles.................................................................................................................................................328 Interaction of the Bipolar Spindle Apparatus with the Cell Envelope ...........................................................328 Cellular positioning of the BSA........................................................................................................................328 Role of small GTPases .......................................................................................................................................329 In the Center of the Bipolar Segregation Apparatus ........................................................................................329 Genesis of Bipolarization.......................................................................................................................................330 Evolutionary Aspects of Cytokinesis ....................................................................................................................330 FtsZ in chloroplasts and mitochondria ...........................................................................................................330 Evolution of FtsZ, ZipA, and FtsA into microtubules, MAPs, and actin? .................................................330 CONCLUSION............................................................................................................................................................330 ACKNOWLEDGMENTS ...........................................................................................................................................331 REFERENCES ............................................................................................................................................................331 solved by the two groups of organisms. This will then serve as an introduction to the (macro)molecular approach, which forms the second part of this review. It will be seen that the distinction between the cellular and macromolecular levels cannot always be consequently maintained. The third part discusses the eukaryotic and prokaryotic solutions to the distribution of genetic material. In this respect it will become clear that there are essential differences between prokaryotes and eukaryotes. Of course, one can choose to emphasize or play down the differences. In this review I will make the point that these differences are important and that recent results as discussed here confirm this point of view. Present research on cytokinesis is rapidly advancing, and several model systems are being used. Animal systems include mammals, Xenopus laevis, Drosophila melanogaster, and Caenorhabditis elegans. Somewhat resembling these systems is the slime mold Dictyostelium discoidium. Yeasts and fungi repre-

INTRODUCTION The cellular basis of life requires cytokinesis after DNA replication and the distribution of the genetic material over two new daughter cells (mitosis). To ensure that these daughter cells are genetically identical, nuclear division and cytokinesis must be well organized with respect to each other in time and space. This comprises cellular logistics in prokaryotes as well as eukaryotes, and it is this aspect of cytokinesis which is the subject of this review. This review consists of three parts. First, I will present a brief overview of cytokinesis on the cellular and subcellular levels. At the same time, I will highlight common problems to be * Mailing address: Swammerdam Institute for Life Sciences, University of Amsterdam, P.O. Box 94062, 1090 GB Amsterdam, The Netherlands. Phone: 31-20-525 5194. Fax: 31-20-525 6271. E-mail: [email protected]. 319

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sent systems where the flexibility of cellular shape is limited by a rigid, though not static, cell wall. These organisms also are distinguished from animal systems by maintaining the nuclear envelope during mitosis. The prokaryotic model organisms are Escherichia coli, Bacillus subtilis, and Caulobacter crescentus. Higher plants will not be discussed here, because they occupy a somewhat different position. CYTOKINESIS AT THE CELLULAR AND SUBCELLULAR LEVELS Organization of the Bipolar Spindle Apparatus A precondition for the cell division process is the bipolar organization of the cell. The first step toward attaining this configuration is the duplication of the centrosome or microtubule-organizing center (MTOC). In higher eukaryotes, but also in Dictyostelium, the MTOC is located near the nuclear envelope. In yeasts like Saccharomyces cerevisiae and Schizosaccharomyces pombe the MTOC is referred to as spindle pole body (SPB). Their respective SPBs are quite different in structure and composition. In S. cerevisiae the SPB is part of the nuclear envelope, whereas in S. pombe it is located in the cytoplasm during interphase and enters the nuclear envelope at the start of mitosis. The various MTOCs all contain ␥-tubulin as part of a microtubule-nucleating apparatus that facilitates radiation of microtubules into the cytoplasm (for a review, see reference 86). Centrosomes and the SPB of S. pombe duplicate at the beginning of mitosis; in S. cerevisiae duplication occurs at the beginning of S phase (for reviews, see references 6 and 37). After duplication, one of the MTOCs moves to a diametrically opposite position in the cell. The early SPB duplication in S. cerevisiae with respect to S. pombe, might be required for the movement of the nucleus into the growing bud (see below). At the onset of mitosis, microtubules become arranged between the MTOCs. Because the nuclear envelope disintegrates during mitosis in higher eukaryotes, the inter-MTOC microtubules are in direct contact with the cytoplasm. By contrast, the nuclear envelope in yeasts persists, thus preventing intracellular contacts of the inter-MTOC (SPB) microtubules beyond the nuclear envelope (unless the nuclear pores serve a communicating function [see below]). Despite minor differences, the essential elements of a bipolar spindle organization, here referred to as the bipolar spindle apparatus (BSA), are shared among all eukaryotes. Location of the Division Plane in Eukaryotes In most eukaryotes the division plane becomes located halfway between the MTOCs of the BSA. Displacement of the BSA by centrifugation in Crepidula eggs was accompanied by displacement of the cleavage plane (Fig. 1a) (reference 91 and references therein). This classic experiment by Conklin has emphasized the potential importance of the cellular envelope region halfway between the MTOCs. Subsequent research has also revealed the importance of astral microtubule-mediated interaction with the nearest cell cortex for BSA positioning in the cell (for a review, see reference 101) (see below). In S. cerevisiae, division site selection is a direct consequence of bud site selection (14). Thus, to place the predetermined

FIG. 1. Position of the BSA relative to the site of cytokinesis. (a) Animal cell. In the right-hand cell, the BSA has been shifted to the periphery by pressing a glass bead onto the mitotic cell. (b) S. cerevisiae. The elongating nucleus positions itself relative to the neck of the growing bud (thick double arrows). (c) S. pombe. Medial ring positioning follows the predetermined site of the nucleus. MT, microtubules.

division plane halfway along the BSA, the nucleus itself has to be moved in a proper orientation in the cell (13) (Fig. 1b). This leads to the question of how the mitotic nucleus becomes properly oriented in a budding cell. A likely mediator is the SPB because it connects the inner and outer surface of the nuclear envelope. Inside the nucleus, the mitotic spindle elongates between the SPBs. Outside the nucleus, astral microtubules radiate to the opposite plasma membrane in the growing bud. Notably, in S. cerevisiae the interaction of astral microtubules with the opposing plasma membrane has been demonstrated (107). Eventually, the BSA orients toward a preexisting septum marker, the bud site (Fig. 1b). By contrast, in S. pombe the division site must be established in relation to the cellular position of the nucleus (Fig. 1c) (13). To do this, a mechanism can be expected which maintains the nucleus in the cell center. Presumably this is achieved, at least in interphase, through the microtubular framework which surrounds the nucleus (38). It can be imagined that this frame-

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properly oriented in the cell. However, bacteria lack MTOCs, and therefore there must be a bacterial equivalent of the eukaryotic MTOC. The above outline is rather general. Below I will embark on a molecular description of cytokinesis in bipolar cells. For eukaryotes this refers to the role of the dynamic cytoskeleton and its many associated proteins. For prokaryotes we need to find whether there is a cytoskeleton-like equivalent. CYTOKINESIS AT THE MACROMOLECULAR LEVEL Cytokinesis and the Cytoskeleton

FIG. 2. Cell cycle phases in a prokaryote and in a eukaryote. In a prokaryote, the bacterial chromosome does not condense prior to segregation. The chromosome separates during its replication. Therefore, the S phase coincides with the M phase. G1, cell cycle phase prior to DNA replication. In E. coli, this phase is denoted B. Its occurrence depends on the strain and on growth conditions. G2, phase between termination of DNA replication and mitosis. This phase is not present in E. coli. S, DNA replication phase.

work also interacts with the plasma membrane, thus positioning the nucleus at a defined position in the cell. In essence, this representation of the two yeasts is not different from that of a cleaving higher eukaryotic cell. In all cases, the final spatial orientation is the same: a defined orientation of the division plane perpendicular to the BSA. What differs is the way in which this orientation is achieved. Location of the Division Plane in Prokaryotes Early light micoscopic data have shown that the E. coli division plane is positioned between segregated chromosomes and halfway between the cell poles (70, 104). DNA compaction after completion of replication, as generally occurs during eukaryotic mitosis, has so far not been demonstrated in bacteria. In fact, there is no need for such a phase because, as demonstrated previously (104) and studied in E. coli in particular, the replicating chromosome becomes distributed over the prospective daughter cells during elongation of the cell (for reviews, see references 32, 103, and 125). E. coli therefore does not have a separate M phase. This is depicted schematically in Fig. 2, where the prokaryotic and eukaryotic cell cycle phases are compared. Preliminary Conclusions The comparison of cytokinesis of eukaryotes and prokaryotes at the cellular and subcellular levels can be summarized as follows. (i) Before cytokinesis, a BSA is formed in eukaryotes. (ii) Orientation of the BSA is facilitated by astral microtubules that interact with the opposing cortex or cell envelope. (iii) The cleavage plane has a defined position between the MTOCs. (iv) S. cerevisiae deviates in the sense that the BSA adjusts its position with respect to the predetermined cleavage site; in all other cases discussed here, the reverse is true. (v) In prokaryotes, cytokinesis occurs halfway between the cell poles. As in eukaryotes, the replicated bacterial chromosome must become

Cytokinetic ring. Cytokinesis in eukaryotes is achieved by a contractile ring associated with the plasma membrane. The ring basically consists of actin and myosin II, and the actomyosin organization resembles that of smooth muscle (for a review, see reference 101). In nondividing cells this ring is absent, and its establishment involves extensive spatial reorganization of the actin filament-based cytoskeleton. Apart from the contractile ring and its association with the cell cortex, the microtubules and the microtubule-associated proteins in the spindle interzone have been invoked as instrumental in the cytokinetic process (for reviews, see references 28 and 101). It is not my aim to compare in detail the cytokinetic process in different eukaryotes; rather, I will attempt to provide an overall “eukaryotic picture.” Doubtless this picture will be basic. However, by focusing on the main aspects, a comparison with prokaryotic cytokinesis will be facilitated. It will be appreciated that the circumference of the cytokinetic ring of E. coli is considerably smaller than its eukaryotic counterpart. Even in a small eukaryote like D. discoidium, there is a 100-fold difference (Fig. 3). In addition, the macromolecular composition of the E. coli cytokinetic ring is essentially different. As mentioned above, two main components of the cytoskeletal framework, actomyosin complexes and microtubules, and their respective associated proteins play a vital role in eukaryotic cytokinesis. Broadly speaking, microtubules help in positioning of the bipolar BSA in the cell and actin filaments are essential for the septation process. An additional feature of eukaryotic cytokinesis is the involvement of members of the Rho family of small GTPases in establishing a local interaction between the cytoskeleton and the plasma membrane. In yeasts this applies to the cell wall, and in higher eukaryotes it applies to the extracellular matrix. It will be seen that although the cell wall is instrumental in carrying out prokaryotic cytokinesis, Rho-like GTPases are probably not involved. Actomyosin and associated proteins. In fungi as well as animal cells, actomyosin complexes play a role in carrying out the septation process. Since such complexes are also present elsewhere in the cell before cytokinesis, the localized rearrangement of the actomyosin network is an important prerequisite for division. Many other actin binding or actin-associated proteins localize to the septum or cleavage plane. One group of such components are septins. In S. cerevisiae, septins are part of a network in the neck of the bud underneath the plasma membrane (11). Septins are indispensable for septation and form a scaffold for the assembly of the actomyosin complex (for reviews, see references: 27, 62, 65, and 88). Several septins have been found in S. cerevisiae,

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with microtubules containing a green fluorescent protein GFP–␣tubulin fusion protein (12) or with microtubules labeled with a dynein-GFP fusion (107). These experiments showed the dynamic instability and orientation of microtubules emanating from an SPB in the developing bud (Fig. 4). The searching behavior of the growing astral microtubules ultimately resulted in a defined placement of the BSA in the cell. In S. pombe, in early mitosis the future septation site is already marked through the formation of the medial ring between the duplicated SPBs (for a review, see reference 34). This ring also contains microtubule components (87) (see below). However, medial ring positioning (in contrast to nucleus positioning) is not dependent on intact microtubules (108). Interestingly, longitudinally arranged microtubules appear to be associated with specific proteins at the extreme ends of interphase cells (Fig. 4). One of these proteins is Tea1p (tip elongation aberrant), which, when mutated, produces T-shaped cells. It has therefore been suggested that Tea1p is required for maintaining the normal rod shape of S. pombe (68). A similar

FIG. 3. Cytokinetic rings in a eukaryote and a prokaryote. In the highly schematized eukaryote, the cytokinetic ring is represented by a vertical white bar. In the prokaryote, E. coli, a cell division protein, FtsQ, has been labeled by its fusion to green fluorescent protein (GFP). Note the difference in the sizes of the two cells. (Copyright T. den Blaauwen and J. Chen.)

and some of these proteins have also been found in other eukaryotes, including animals. Septins show GTPase activity, although the functional significance of this observation is still unclear (27). In S. pombe, a Mid1p ring precedes the actin ring. However, whereas the Mid1p ring arises independently of the actin ring, both are dependent on actin-associated proteins (108). Interestingly, Mid1p localizes to the nucleus in interphase, suggesting that it leaves the nucleus and then forms a ring at the onset of mitosis (108). This fits, but does not explain, the observation that the central position of the nucleus is important in establishing the site of cytokinesis in S. pombe (38, 39). This location makes sense because in eukaryotes that maintain the nuclear envelope during mitosis, it is the nucleus that has to be located relative to the site of septation and not the mitotic spindle. By contrast, when the nuclear envelope is broken down, spindle components might directly interact with the medial cortex. The number of proteins being discovered that bind actin (Abp proteins), apart from myosins, is steadily increasing, which not only stresses the importance of the actomyosin network but also points to the complexity of the regulation of its functioning. Microtubules and SPBs. Of the three classes of spindle microtubules, namely, kinetochore-bound microtubules, centrosome-connecting microtubules, and astral microtubules, the last, because of their interaction with the cortex, have received the most attention recently. This interaction makes the cortex perhaps a prime determinant of the bipolar organization of the cell. Two examples, in S. cerevisiae and S. pombe, underline this. In S. cerevisiae, elegant in vivo studies have been carried out

FIG. 4. Interaction between microtubules and growing poles in S. cerevisiae and S. pombe. (a) In S. cerevisiae, astral microtubles explore the poles while the nucleus moves into the bud. The motor protein dynein travels along microtubules and pulls at the dynactin molecules at the cell cortex. Reprinted from reference 12 with permission of the publisher. (b) Growing microtubules deposit Tea1p at the poles. Adapted from reference 69, with permission of the publisher, MT, microtubules.

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suggestion has been made for Pom1p, a kinase, which localizes to the cell poles as well as the septum (3). Tea1p localization does not require Pom1p, whereas Pom1p needs Tea1p, suggesting that Pom1p acts downstream of Tea1p at the cell poles (3). Tea1p persists at the poles during mitosis, whereas the microtubules at the two poles shrink (68). Whether subsequent astral microtubules become associated with Tea1p during mitotic anaphase has not yet been studied. In fact, the interaction of astral microtubules with the polar cortex region in living S. pombe has received less attention than in S. cerevisiae (however, see reference 17). Very recently, a kinesin-like protein, Tea2p, has been detected at the cellular poles (8). Tea1p localization appeared dependent on Tea2p, and one speculation was that Tea1p represents cargo for the motor protein (Tea2p) to be transported to the cell poles along microtubules (8). Protein kinases that localize to the SPB as well as to the medial ring have been detected in S. pombe. Examples include a polo-like kinase Plo1p (4); an IQGAP-related protein, Rng2p (23) (see below); and an SPB kinase, Sid2p (109). The dual localization of these proteins has been interpreted to mean that the SPB functions as a signaling site for cytokinesis, presumably with the aid of microtubules (reference 108 and references therein). It would thus seem that the interphase organization of the microtubular framework, including polar interactions, contributes to the positioning of the nucleus in the cell center at the onset of mitosis. Microtubule-associated motor proteins. It is well known that microtubules serve as roadways for plus-end-directed (kinesins) and minus-end-directed (dyneins) motor proteins, thus allowing a polarized arrangement and transport of cellular components. Astral microtubules presumably interact with the cortex in animal cells with the aid of minus-end-directed proteins (for reviews, see references 2 and 61). As mentioned above, such interaction has also been implied for the astral microtubules in the the growing bud of S. cerevisiae (Fig. 4) (107). The astral microtubule-cortex interaction in animal cells is thought to predetermine the cellular position of the BSA and eventually the location of the cleavage plane (for a review, see reference 101). As pointed out above, the medial cortex becomes irreversibly committed to cytokinesis (even in the absence of a BSA), provided that astral microtubule-cortex contact lasted long enough (for a review, see reference 92). Small GTPases and the cellular envelope. Small GTP-binding proteins and heterotrimeric G proteins represent molecular switches that influence numerous processes related to the dynamic cellular organization (2, 61). The monomeric small GTPases include subfamilies such as Ras and Rho (41). Their functioning as switches is based on general principles. Basically, the switch involves an alternation between two conformational states which are defined by the binding of either GTP or GDP. Nucleotide binding is affected by activators, effectors, and inhibitors, which in turn are influenced by additional components. In the example shown in Fig. 5 (“Ras protein paradigma” [41]), the GTP-bound form activates an effector. Inactivation occurs through hydrolysis of GTP, resulting in the inactive GDP-bound form. In vivo hydrolysis is controlled by GTPase-activating proteins (GAPs). The GTP-bound form arises through a GDP-GTP exchange reaction, which is catalyzed by a guanine nucleotide exchange factor. In S. cerevisiae, Rho small GTPases include Cdc42p and they

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FIG. 5. The Rho-GTPase molecular switch. The conformation of the GTPase depends on whether GTP or GDP is bound. The GTPbound form functions through an effector. This interaction is abolished when GTP is hydrolyzed with the aid of GAP. A guanine nucleotide exchange factor (GEF) catalyzes the production of active Rho-GTP. Adapted from reference 41.

function in establishing cell polarity for focused bud growth and septation. This polarizing effect requires dynamic rearrangement of the actomyosin cytoskeleton toward the plasma membrane, to which Cdc42p is attached through a membrane anchor. The activity of Cdc42p is modulated by a GEF (Cdc24p) as well as by GAPs (Bem3p and Gga1p) (for a review, see reference 88). Activated Cdc42p stimulates yeast homologs of p21-activated kinases (PAKs) such as Ste20p and Cla4p (22), which in turn phosphorylate myosin II heavy-chain head domains. In this example, the myosin II heavy chain was isolated from Dictyostelium whereas the kinases were purified from yeast (Ste20p and Cla4p) and rat brain (PAK) (reference 128 and references therein). Note that S. pombe contains myosin II heavy chains in the actomyosin ring (50). Remarkably, myosin II appeared dispensable in substrateanchored Dictyostelium cells, whereas this was not so for cells in suspension. It has been speculated that the extent to which polar regions of a dividing cell are fixed affects the establishment of a “proper gradient of cortical tension” as required for cytokinesis (for a review, see reference 30). In Xenopus embryos, the GTPases Rho and Cdc42 promote furrow ingress during cytokinesis. Whereas Rho appeared to be involved in the organization of the actin cytoskeleton, the role of Cdc42 was less clear. In the latter case, the possibility was raised that Cdc42 interacts with the septin network (20). The role of Rho in Xenopus cytokinesis is reminiscent of the earlier observation that fibroblast Rho is instrumental in the assembly of focal adhesions (96). The interaction of cytoplasmic components with the plasma membrane is of particular importance, because the plasma membrane has to contract with the aid of an actomyosin machinery. Not surprisingly, lipid components also appear to affect the organization of the actomyosin skeleton. For instance, in Xenopus extracts actin assembly could be induced by phosphoinositides in the presence of small GTP-binding proteins (64) (see below). Similarly, phosphatidylinositol-4,5-biphosphate appeared to dissociate actin-Abp complexes (gelsolin) in permeabilized human platelets, thus allowing polymerization of actin into a subcortical actin network. The Abp proteins also mediate the interaction with the plasma membrane (integrins)

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In D. discoideum, the IQGAP-like protein GAPA appeared to be required for the completion of cytokinesis, because myosin II was still deposited in the cleavage furrow in a gapA mutant (1). Also, in S. cerevisiae gene disruption of the IQGAPlike gene (Cys1/Iqg1) allowed the formation of a myosin II-containing ring (58). However, accumulation of actin filaments in the cytokinetic ring appeared to be Cyk1p/Iqg1p dependent (24, 58). These observations fit the idea that IQGAPs play a role in cellular actomyosin architecture, presumably (at least in part) through the GRD domain. It has also been suggested that IQGAPs interact with Cdc42p to establish actomyosin ring assembly (23, 58). Bacterial Cytokinesis

FIG. 6. Domain composition and homology of IQGAP-related proteins from various organisms. The percentages refer to the conservation of domain composition compared to mammalian IQGAP1. CH, calponin homology domain; IR, internal repeats; WW, SH3-mimicking domain; IQ, calmodulin binding motifs; GRD, RasGAP-homology domain. For further details, see reference 65. Adapted from reference 65 with permission of the publisher.

at focal adhesions, which influences the organization of the extracellular matrix in animal cells (2, 61). Until now, the extracellular matrix has received little attention as a possible factor in cytokinesis. By contrast, the yeast equivalent of the extracellular matrix, the cell wall, is clearly instrumental in effecting cytokinesis. The same consideration applies to prokaryotes. Rho1p of S. cerevisiae is involved in the regulation of cell wall synthesis, because it activates 1,3-␤-glucan synthase, which makes a major polymer (1,3-␤-glucan) of the yeast cell wall (21, 89). Rho1p, like Cdc42p, also regulates the reorganization of the actin cytoskeleton, which in the former case is accomplished by the interaction with the actin-binding protein profilin as mediated through Bni1p and Bnr1p (46). This brief survey indicates that the regulation of the organization of the actin skeleton toward the plasma membrane and beyond (cell wall and extracellular matrix) is in principle conserved in a variety of lower and higher eukaryotes. IQGAPs. A cytokinetic role of IQGAP-like proteins has been demonstrated so far for D. discoideum, S. cerevisiae, and S. pombe. These proteins were first discovered in humans (82, 120) and were first considered to be GAPs (for a recent review, see reference 65). IQGAPs contain a number of domains (Fig. 6), which include CH (calponin homology domain which putatively binds to actin), IQ (calmodulin-binding motifs) and GRD (GAP-related domain) domains. IQGAP-like proteins have been found in other organisms, but the extent to which the different domains have been retained varies considerably (65). Whereas the D. discoideum IQGAP possesses only the IQ and GRD domains, the S. cerevisiae and S. pombe IQGAP-like proteins resemble the human protein in organization.

Cytokinesis in E. coli. Traditionally, cytokinesis has been termed cell division in prokaryotes. In E. coli, division is basically a constriction process, whereas in B. subtilis, there is an ingrowing septum like that in S. pombe. Constriction in E. coli involves three envelope layers, which are, from outside to inside, the outer membrane, the peptidoglycan layer, and the cell membrane. The peptidoglycan layer is embedded in the socalled periplasm and serves as an exoskeleton to the bacterial cell. Its local synthesis at the site of constriction is indispensable for the division process, as are cytoplasmic polymers (for reviews, see references 43 and 76) (see below). Several proteins are known that are specifically involved in the division process (for reviews, see references 63, 66, 76, and 99). All of them have been detected as temperature-sensitive mutants that filament at the nonpermissive temperature, and therefore many of the proteins have the prefix Fts. The majority of the Fts proteins are anchored to the cell membrane; their main domains protrude into the cytoplasm or into the periplasm (Fig. 7). All cell division proteins were previously localized to the site of division by fluorescence microscopy (Fig. 3). The use of appropriate mutants and microscopy has likewise resulted in a tentative temporal assembly line of the division apparatus, called the divisome. Assembly of the divisome starts with the positioning of an FtsZ ring in the cell center. The ring is

FIG. 7. Topology of E. coli cell division proteins relative to the plasma membrane. The numbers in the molecules reflect the molecular masses in kilodaltons. With the exception of FtsK and FtsW, the membrane-bound proteins contain a single membrane anchor. The way in which the various proteins interact with each other is not known. pm, plasma membrane; pg, peptidoglycan. Reprinted from reference 76a with permission of the publisher.

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FIG. 8. Schematic representation of the FtsZ ring and its associated subassemblies. The subassemblies are supposed to contain proteins involved in peptidoglycan synthesis in addition to the cell division proteins depicted in Fig. 7. Reprinted from reference 76 with permission of the publisher.

stabilized by FtsA and ZipA (Z-interacting protein A) (40, 73). Subsequently, FtsK, FtsW, FtsQ, FtsL, and FtsN become recruited to the cytokinetic ring (for a review, see reference 66). The number of Fts proteins per divisome is on the order of thousands for FtsZ, and on the order of hundreds or even less for most of the others. Thus, presumably, the cytokinetic FtsZ ring is composed of an FtsZ backbone to which other Fts proteins are attached at a limited number of sites, thus forming divisome subassemblies (Fig. 8). In recent years, increasing attention has been focused on the cytoplasmic division protein FtsZ (for reviews, see references 26 and 66). It is a GTPase and can polymerize in vitro as well as in vivo into filamentous polymers. FtsZ bears structural resemblance to eukaryotic tubulin (for a review, see reference 24). However, microtubular structures composed of FtsZ have not been detected in prokaryotes. Little is known about the function of most of the division proteins. An exception is FtsI, which is a penicillin-binding protein, also denoted PBP3 (for a review, see reference 77). It plays a role in division-specific peptidoglycan synthesis. As a consequence, components of the peptidoglycan-synthesizing machinery may be present in the divisome. Division-specific peptidoglycan synthesis starts almost simultaneously with the positioning of FtsZ near the cytoplasmic membrane at the cell center (16), suggesting a tight link between cytoplasmic FtsZ polymerization and periplasmic peptidoglycan synthesis (75). The chemical nature of the link is not known. What is the cytokinetic motive force? A contractile mechanism based on an actomyosin-complex, like that in eukaryotes, appears unlikely in prokaryotes. Motor proteins such as myosins, kinesins, and dyneins have not been convincingly detected in E. coli. Since the diameter of the E. coli divisome decreases during division, it appears plausible that regulated depolymerization of the FtsZ ring is an important agent, next to the ingrowing leading edge of the peptidoglycan layer. In B. subtilis, a protein, EzrA, has been identified that modulates the frequency and position of FtsZ ring formation (55). It has

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therefore been suggested that EzrA plays a regulatory role in FtsZ depolymerization during ring contraction. Bipolar organization of the bacterial replicating chromosome. Mitosis in eukaryotes is an active process, encompassing dynamic cytoskeletal structures and various motor proteins. It even can proceed in a cell-free system (2, 61). By contrast, the small size of a bacterium (Fig. 3) has precluded a description of its partitioning apparatus in macromolecular terms. It also explains why progress in this field has been made only very recently (see below). It should be remembered that E. coli as well as B. subtilis has a circular genome which replicates bidirectionally from a fixed origin of replication toward the socalled terminus diametrically opposite the origin. This situation is quite different from that of eukaryotes, where there are many origins per chromosome and where defined termini are probably not present (Fig. 9). So far, for prokaryotes a dominant view has been that the interaction of the bacterial chromosome with the elongating cell envelope is instrumental in separating newly replicated chromosomal domains. Recent reports based on fluorescence microscopy images of labeled DNA segments or of proteins closely associated with the origin of replication did, however, suggest that prokaryotes (E. coli [33, 78, 81, 97], B. subtilis [31, 57, 119], and C. crescentus [47, 71]) contain a bipolar partitioning apparatus. This was deduced from the dynamic behavior of the duplicated origin of replication. The terminus of DNA replication in E. coli has also been localized, and its cellular position could be compared with that of the origin. At one stage of DNA replication, for instance, the origins had moved apart whereas the terminus still persisted at the future site of division (33, 78, 81). In E. coli, the more or less constant distance of the bacterial chromosome (115) and of the origins of replication (78, 97) to the cell poles has been interpreted to mean that replicating chromosomes move apart coincident with cell elongation (96, 122). By contrast, time-lapse images of fluorescent foci com-

FIG. 9. Replicating eukaryotic and prokaryotic (E. coli) chromosomes. A eukaryotic chromosome contains numerous bidirectional replicating units, each containing its own origin of replication. E. coli replication starts at a defined origin and proceeds bidirectionally. An E. coli chromosome does not contain a centromere or telomeres.

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posed of GFP-LacI fusions bound to tandem lac operators at specific chromosomal sites in E. coli (33) and in B. subtilis (118) have indicated that these foci can move independently of cell elongation. Foci of labeled proteins which colocalize with the origin of replication of B. subtilis (the ParB-like Spo0J protein [56]) also separate in conjunction with the duplicated origin independently of cell elongation (31). This has been interpreted to mean that an active agent (“mitotic-like” [31, 106]) helps in separating the nascent daughter chromosomes. It would thus seem, that genes occupy a cellular position according to two levels of organization. On one level, their position is linked to the chromosome partitioning process, in the sense that the replicated origins and nearby genes are arranged in a bipolar fashion. On the other level, genes might move within the confinement of a nucleoid compartment (98). Chromosome segregation. Once decatenated, how do the duplicated chromosomes assume their new positions in the prospective daughter cells and how does FtsZ ring formation correlate with bacterial chromosome partitioning? It has been found that, within experimental error, FtsZ-ring formation in E. coli coincides with termination of DNA replication (16). Because the nucleoid borders (115) as well as the duplicated oriC origins (78, 97) maintain a constant distance from the cell poles in an elongating cell, the question can be asked whether the replicated termini become pushed apart by an active mechanism. Conceptually, one could invoke cytoplasmic components and/or the invaginating cell envelope. For FtsZ, the indications for a role in chromosome partitioning are twofold. First, a delay in nucleoid segregation has been inferred in FtsZ-depleted cells (113), and second, nucleoid segregation appeared affected in an ftsZ temperature-sensitive mutant at the nonpermissive temperature (45; C. L. Woldringh, personal communication). Evidence for an envelope role also arose from the interesting observation that separation of the replicated chromosomes is delayed in an ftsI temperature-sensitive mutant at the nonpermissive temperature (45, 110) or after impairment of FtsI (PBP3)-specific peptidoglycan synthesis by specific penicillins (45). This suggests that division-specific envelope synthesis contributes to the partitioning of daughter chromosomes. Perhaps the above-mentioned role of FtsZ in chromosome partitioning is indirect, because it is plausible that impairment of membrane-associated FtsZ affects division-specific peptidoglycan synthesis. Another actor possibly involved in DNA segregation is the largest cell division protein, FtsK (147 kDa) (Fig. 7). Its N-terminal part is needed for localization at midcell (19, 59, 116, 131), whereas its C-terminal might play a role in DNA segregation (59). The C-terminal part of FtsK resembles the B. subtilis protein SpoIIIE (59), which is thought to act during the transfer of spore DNA to the mother cell (129). Does DNA condensation play a role during chromosome segregation? Like eukaryotes, prokaryotes possess structural maintenance of chromosomes (SMC) proteins. In eukaryotes, these proteins are involved in mitotic chromosome condensation and in chromatid cohesion. These activities are carried out with the help of other proteins, and the complexes have been termed condensin and cohesin, respectively. In these multiprotein complexes, different SMCs are present and occur as heterodimers (for reviews, see references 5, 42, and 70). The E. coli SMC-like protein, MukB, is a 177-kDa protein

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which affects chromosome segregation, as witnessed through dispersion of nucleoid structure and the formation of DNAless cells in temperature-sensitive mukB mutants (reference 80 and references therein). It occurs as a homodimer in the cell (79) and, like its eukaryotic counterpart, is a rod-like molecule with globular ends (reference 70 and references therein). The N-terminal part resembles dynamin and can bind ATP and GTP, whereas the role of the C-terminal part is less clear (79). MukB can also bind DNA. It should be mentioned that the structure of MukB resembles that of SMCs, whereas its amino acid sequence does not (70). Recently, it was shown that the N-terminal part of MukB (containing the nucleotide-binding domain) can bind to polymerized FtsZ in the presence of GTP (60). This suggests that chromosome segregation includes a MukB-mediated connection between DNA and the FtsZ ring. However, immunofluorescence microscopy of MukB revealed its confinement to the nucleoid and not to the FtsZ ring (T. den Blaauwen et al., submitted for publication). The B. subtilis SMC-like protein Smc bears more sequence resemblance to its eukaryotic counterparts than does MukB (70). Like MukB, it affects chromosome segregation (8, 72), and immunofluorescence microscopy has shown that it occurs at poles and near the nucleoid (8, 35). Its cellular location, therefore, does not coincide with that of MukB in E. coli. Are there prokaryotic condensins and cohesins? It should be stipulated that prokaryotes (including archaebacteria) sequenced until recently contained either a single smc gene or none at all (for a review, see reference 42). This suggests that in prokaryotes a condensin-cohesin function, if it occurs, may reside in one and the same multiprotein complex. Interestingly, recent evidence showed that MukB forms a complex with MukE and MukF (130). However, the notion that bacterial chromosomes segregate while being replicated (32, 122) precludes a cohension function, at least in E. coli. Likewise, there is no evidence that bacterial chromosome become compacted in the same way as required for eukaryotic mitosis (however, see below). The location of MukB in the nucleoid (den Blaauwen et al., submitted) and the dispersion of the nucleoid in mukB mutants (80) suggest a nucleoid-scaffolding function for MukB, as has also been suggested for condensin in eukaryotic mitotic chromosomes (reference: 5 and references therein). In fact, as shown recently, negative supercoiling of the E. coli nucleoid appeared to be reduced in a mukB mutant (121). In line with this observation, suppression of the mukB phenotype occurred through a mutation in topA (encoding topoisomerase I), which led to excess negative supercoiling (102). MukB may aid the chromosome segregation process by local DNA condensation, thus providing a pulling action on the replicating nucleoid. How does E. coli know where to divide? The simple question of how E. coli knows where to divide has generated much speculation. One view is based on the observation that cytokinesis takes place between duplicated nucleoids when they are sufficiently separated (nucleoid occlusion). Thus, the location of the division plane is directly related to the cellular position of the replicating bacterial chromosome (74, 124). Whereas the original nucleoid occlusion model was based on observing cellular constrictions as such, recent studies have confirmed that the positioning of the FtsZ ring is affected by the nucleoid in a similar

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DIFFERENT SOLUTIONS What Is the E. coli Equivalent of the Bipolar Spindle Apparatus?

FIG. 10. Time-lapse images in seconds (numbers) of GFP-MinD oscillations in living E. coli cells. Fluorescent GFP-MinD appears as bright polar regions. The cells to the right were imaged with differential interference contrast. For details, see reference 94 (reprinted with permission of the publisher).

way (36, 112). The observed coincidence of termination of DNA replication and FtsZ ring formation (16) also fits the model. The selection of the division plane has achieved a new dimension through the observation of oscillation of GFP-labeled MinC (44), MinD (94, 100), and MinE (29) between the two cellular poles (Fig. 10). These proteins have acquired the prefix Min because when they are mutated, polar divisions occur that produce DNA-less minicells. Thus, E. coli contains three potential division sites, the central one and one at each pole. According to a model (15), the combination of MinCD inhibits polymerization of FtsZ at potential division sites unless MinE is present. That is, under wild-type conditions, MinE prevents the inhibition of FtsZ polymerization at the cell center. In line with the model, a MinE ring has been observed at the cell center, independent of FtsZ (93). As pointed out by Raskin and de Boer (94), the abovementioned Min protein-based oscillator(s) might serve as a measuring device leading to the definition of the cell center. Note that the timescale of the oscillations is on the order of minutes, showing that they take place many times during a division cycle. Is there a link between nucleoid occlusion and MinE ring positioning? MinE-GFP rings have been observed on top of nucleoids, whereas this was not the case for FtsZ rings in parallel samples (112). Since FtsZ polymerization is inhibited by a combination of MinC and MinD (and relieved by MinE) and also by the presence of a nonsegregated nucleoid, it has been suggested that FtsZ ring assembly is subject to two negative topological regulators (112). Finding how these regulators interact will be a great challenge. To the best of my knowledge, a cell pole-to-cell pole oscillator has not been described for eukaryotic cells. It will be important to know whether such an agent reflects a general princple in a cell’s search for its prospective site of fission.

Mitosis in eukaryotic cells requires DNA compaction into discrete chromosomes after the cells have traversed through the S and G2 cell cycle phases. In prokaryotes, no distinction can be made between interphase and mitosis. DNA segregation goes hand in hand with replication (122, 125, 127). Therefore, prokaryotes have no mitosis in the classical (eukaryotic) sense (Fig. 2). Even so, this does not detract from the concept that cytokinesis in prokaryotes requires a bipolar cellular arrangement of the replicating or replicated chromosome. How can the macromolecular differences be assessed? I will start with the well-known eukaryotic situation and enumerate some perhaps obvious aspects (2, 61). (i) DNA replication occurs through numerous origins (Fig. 9). (ii) After completion of DNA replication, the sister chromosomes (chromatids) stay together until mitotic anaphase. (iii) The replication origins play no role in chromatid separation. (iv) Chromatid separation is preceded by integral chromosome compaction. (v) A bipolar spindle apparatus is formed in between duplicated MTOCs. (vi) Sister chromosome separation requires the dissolution of a so-called cohesin complex (reference 117 and references therein) (vii) Chromatid segregation is effected by shortening of microtubules between the kinetochore and MTOC. (viii) The kinetochore binds to specific nucleotide sequences (centromere). I will now list, with some generalization, the prokaryotic solution. (i) DNA replication occurs through one origin only (Fig. 9). (ii) The bacterial “chromatids” do not stay together; they move apart during the DNA replication process. (iii) Nucleotide sequences near the origin of replication provide binding sites for proteins that may assist in DNA separation, i.e., in maintaining a constant distance of the origin to the cell pole (note that B. subtilis is an exception [105]). (iv) No microtubules or kinetochores are present in bacteria. (v) Apart from SMClike proteins, classical motor proteins resembling myosin, dynein, and kinesin have not yet been detected in prokaryotes. As suggested above, one can consider the origin of replication and/or associated proteins to be the eukaryotic centromere analogue (reference 33, and references therein). The prokaryotic terminus of DNA replication would then be the equivalent of the telomere. In a general mechanistic sense, this appears plausible. However, in macromolecular terms, this comparison does not hold. Eukaryotic origins of DNA replication play no role in mitosis, and telomeres have no functional relationship to the prokaryotic terminus of replication. The bacterial analogue of the MTOC is more difficult to define. During mitosis, centromeres (while bound to kinetochores) approach MTOCs. Does the E. coli origin of replication, as a centromere analogue, approach something? Because the average distance of duplicated oriC origins from the cell poles remains constant during most of the DNA replication cycle while the cell elongates (78, 97), the cellular poles might be considered the MTOC analogues. Consequently, according to this reasoning, the E. coli centromere analogue (oriC) and its MTOC analogue (the cellular pole) coincide. However, it has also been shown that individual GFP-labeled gene regions can move independently of cell elongation (reference 33 and

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eukaryote. In fission yeast, directionality of cell wall assembly is probably aided by a polarized cytoskeleton. In S. pombe, microtubules are arranged in such a way that their positive ends point to the respective poles. The presumed structural communication between the poles might thus be mediated through a defined cellular structure (Fig. 4). How does E. coli fit in this scheme? In the absence of a classical cytoskeleton in E. coli, i.e., one located in the cytoplasm, it is not immediately clear how polar communication is achieved or whether it is at all relevant. However, as mentioned in a previous section, recent evidence has shown that MinC and MinD can oscillate between poles (Fig. 10). It would thus seem that the absence of a cytoskeleton in a small prokaryote is compensated for by an oscillation mechanism presumably based on protein diffusion. Interaction of the Bipolar Spindle Apparatus with the Cell Envelope

FIG. 11. Comparison of cell wall extension in S. pombe and in E. coli. S. pombe extends at the poles, presumably aided by the polarized microtubular framework. In E. coli, the poles are inert with respect to cell wall extension and cell elongation takes place between the poles. Here the poles are not connected through a cytoskeletal structure in the cytoplasm. A diffusible component oscillates between the poles instead. MT, microtuble. For further details, see the text.

see above), which suggests some autonomy with respect to the mobility of subcompartments of the segregating nucleoid. On the other hand, the E. coli nucleoid as a whole seems to separate along with cell elongation (115). Thus, globally, it would seem that the replicating and simultaneaously segregating bacterial chromosome, with its origins directed toward the cell poles, represents the E. coli equivalent of the BSA.

Cellular positioning of the BSA. In previous sections I have mentioned the interplay of eukaryotic cytoskeletal elements with the envelope (cortex, plasma membrane, and cell wall) of the cell. These interactions referred to the behavior of astral microtubules while connecting the separating MTOCs to the nearest cortex and to the cytokinesis process as such in the cell center. All these interactions will serve to confine the eukaryotic BSA in its proper cellular position. What keeps the bacterial chromosome in its place in the absence of eukaryotic cell-like cytoskeletal structures? It has been hypothesized that the confinement of the nucleoid is the result of the balancing of two opposing forces (123). On the one hand, compaction is achieved by DNA supercoiling, DNA-binding proteins, and phase separation of cytoplasm and nucleoplasm (126); on the other hand, DNA transcription at the cytoplasmic surface of the nucleoid is thought to loosen its structure, in particular when DNA loops are directed to the cytoplasmic membrane by the process of coupled transcription, translation, and membrane protein insertion (123) (Fig. 12).

Exploring the Poles Comparison of E. coli and S. pombe with respect to polarity and envelope extension will also serve to illuminate basic structural differences between prokaryotes and eukaryotes (Fig. 11). Cell wall extension in S. pombe occurs at the poles, either at one or at two poles (for a review, see reference 48). Presumably, building blocks of the growing poles are carried to their destination with the aid of a microtubule-based transport system. As mentioned above, Tea proteins might direct microtubules to their proper polar location. By contrast, E. coli poles are largely inert and envelope extension takes place through intercalation of peptidoglycan precursors in the lateral wall (for reviews, see references 43, and 76). Here, gene products needed for envelope assembly (membrane-anchored PBPs) find their way through coupled transcription-translation-membrane insertion. It might be speculated, that in the latter case the topological arrangement of genes being transcribed is directly related to the pattern of envelope assembly (10, 76) (Fig. 12). Clearly, the spatial separation of transcription and translation by a nuclear envelope precludes this possibility in a

FIG. 12. Cotranslational insertion of membrane proteins in the plasma membrane of E. coli. OM, outer membrane; PM, plasma membrane. (Copyright C. L. Woldringh.)

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FIG. 13. Schematic model of E. coli nucleoid segregation. The nucleoid is transiently connected to the plasma membrane through transcription loops. The model refers to slowly growing cells, where segregation takes place in the long axis of the cell. Reprinted from reference 127 with permission of the publisher.

This would mean that DNA transcription loops serve as structural mediators between the nucleoid and the envelope. In fact, the temporal association of these loops with the elongating rod-shaped cell has been invoked as a mechanism to achieve the gradual separation of the replicating bacterial chromosome (Fig. 13) (123). Is there a bacterial equivalent for astral microtubules? In E. coli, the distances of the duplicated origins of replication (78, 97) and of the nucleoid borders (115) to the cell pole remain more or less constant during the cell cycle. How are this constant distances maintained? One can think of a protein framework attached to the origin of replication or to regions flanking the origin, which extends toward the cell poles. The protein composition of this putative framework is unknown, although one might think of proteins that can form protofilaments, at least in vitro. One might consider FtsZ and EF-Tu (the E. coli ribosomal elongation factor) as potential components. These two components are abundant proteins, with many more molecules present than would be required for cell division or protein synthesis, respectively (reference 76 and references therein). Note that transcription loops as potential structural mediators (Fig. 12) are probably not in contact with the polar envelope. The reason is that the poles are considered inert with respect to envelope synthesis, since this process takes place at the lateral sides (Fig. 11). In E. coli, proteins involved in peptidoglycan assembly are part of the divisome (43, 75, 76). One of these proteins is FtsI, and it was mentioned above, that impairment of this protein delays nucleoid separation (45). Interaction of the E. coli BSA with the cellular envelope is thus twofold: (i) a constant distance is maintained between the origin region and the cell pole, and (ii) defects in cell division proteins affect nucleoid segregation. Role of small GTPases. In a previous section, membraneanchored Rho-like GTPases were mentioned as mediators between the eukaryotic cytoskeletal organization and assembly of the cell envelope, be it the extracellular matrix or the cell wall. An example was Rho1p of S. cerevisiae, which is involved in activation of 1,3-␤-glucan synthase (21, 89) and also in the reorganization of the actin cytoskeleton (46). So far, the only known protein of the E. coli divisome exhibiting GTPase activity is FtsZ. However, tubulin-like FtsZ does not belong to the group of Rho-like GTPases. With respect to switches in E. coli, the “Ras paradigma”

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does not seem to apply. E. coli response regulator proteins, which are part of so-called two-component regulatory systems, function as switches through protein phosphorylation and dephosphorylation in this organism. An example is the regulation of chemotaxis. Whether such two-component regulatory systems play a role in E. coli cytokinesis is not clear. However, evidence is accumulating for a the existence of a transcription factor (CtrA) in C. crescentus which functions as a cell cycle transcriptional regulator. Cell cycle-dependent processes like DNA replication, DNA methylation, and flagellar biosynthesis appear to be controlled by CtrA in this organism (90). Later, it was found that increases in cellular CtrA concentration reduced ftsZ transcription whereas decreases led to the opposite effect (49). Whether a similar global cell cycle regulator functions in E. coli is not known. In the Center of the Bipolar Segregation Apparatus To allow redistribution of duplicated chromosomes over the new daughter cells, chromosomes become aligned in the center of the BSA during eukaryotic metaphase. In E. coli, as discussed above, this refers to the distribution of genes rather than to that of chromosomes. In the classic model of DNA replication, the polymerase holoenzyme travels around the circular chromosome in two directions. Alternatively, as originally proposed by Sueoka and Quinn (111) and by Dingman (18), the DNA to be replicated passes through a stationary structure or replisome. The term “replisome” has been defined as a supramolecular assembly “with helicases, primosome, and polymerase holoenzyme operating coordinately at the replication fork” (52). Indeed, recent experiments indicate that DNA replication takes place in the cell center (in E. coli [51] and B. subtilis [53, 54]). It should be mentioned that the idea of a local

FIG. 14. Bipolarization in a eukaryote and in a prokaryote. In a eukaryotic cell, the compacted chromosomes are arranged in the metaphase plate, which is symmetrically located between the MTOCs. In a prokaryote like E. coli, the replisome is symmetrically located between the duplicated origins (oriC). In this representation, the oriC region of the chromosome and associated proteins are analogues of the MTOC. However, in E. coli, no distinction can be made between the centromere and MTOC functions. Note also that the chromosomes have completed replication in the metaphase plate whereas it is the replisome, active in DNA replication, that is centrally located.

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aggregation of replisomes had already gained some popularity in the eukaryotic field some time ago (85). Of course, in this case the fixed replisome has no relevance for the central position of the metaphase plate. However, a central cellular position of the prokaryotic replisome would collect the genes to be replicated in the cell center before being distributed over the new daughter cells. In this sense, the prokaryotic replisome is the analogue of the eukaryotic metaphase plate (Fig. 14). Genesis of Bipolarization How does bipolarization arise? It obviously needs the duplication of something, and it is directly related to the duplication of the genetic material. In E. coli, duplication of the origin of replication is the first step. Subsequently, the two origins move apart during cell elongation. In this way, bipolarity is created in E. coli. By contrast, duplication of a eukaryotic genome does not suffice to establish a bipolar cell. Duplication of the MTOC is required as a prelude to mitosis, and it is here that the dynamic cytoskeleton comes to the fore. Eukaryotes have thus created a higher level of organization, where the MTOC has taken over the putative segregating function of the prokaryotic origin of replication and its associated proteins (Fig. 14). As mentioned above, in E. coli DNA strands become distributed over new daughter cells, whereas in eukaryotes this applies to a set of sister chromatids. Evolutionary Aspects of Cytokinesis Evolution of cytokinesis can be considered on two levels of cellular organization. First, it can be asked whether the eukaryotic cytokinetic ring evolved from its prokaryotic counterpart. Second, it could be asked whether chloroplasts and mitochondria fission in a prokaryotic way. This question arises because according to the endosymbiosis hypothesis, these organelles are of prokaryotic origin. I will start with the latter topic (for reviews, see references 26 and 84). FtsZ in chloroplasts and mitochondria. Initial attempts to find FtsZ in mitochondria have met with little success. By contrast, in chloroplasts of Arabidopsis thaliana two types of FtsZ proteins have been found. They are nucleus encoded; one of them, FtsZ1, appears to be located in the stroma of the chloroplast, whereas the other one, FtsZ2, occurs the outside of the chloroplast (83). It has been suggested that FtsZ1 would be equivalent to the prokaryotic FtsZ in aiding constriction whereas the external FtsZ2 would carry out a squeezing role (26). In this sense, FtsZ2 would assume a role which in grampositive and gram-negative bacteria is performed by the ingrowing peptidoglycan (75, 76). Very recently, FtsZ has been detected in mitochondria of a lower eukaryote, the brown alga Mallomonas splendens (7). In mitochondria of yeasts and higher eukaryotes, fission, in the absence of FtsZ, is accomplished with the aid of dynamin (26, 84). As mentioned in a previous section, MukB from E. coli and SMC-like protein from B. subtilis each contain a domain that resembles dynamin. In E. coli, MukB and FtsZ do interact (60), suggesting that MukB is, one way or another, involved in bacterial cytokinesis. One might speculate that in the course of

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evolution, the loss of FtsZ provided for a more dominant role of dynamin-like proteins in the fission of eukaryotic organelles. Evolution of FtsZ, ZipA, and FtsA into microtubules, MAPs, and actin filaments, respectively? Until now, little, if anything, was known about the evolution of the eukaryotic cytokeleton. Because of the resemblance of FtsZ to tubulin, it has often been suggested that microtubuli arose from FtsZ (reference 25 and references therein). Interestingly, ZipA (Fig. 7) appears to contain sequence elements in its N-terminal cytoplasmic domain resembling those of eukaryotic microtubule-associated proteins (MAPs) (95). In vitro, ZipA was found to promote bundling of FtsZ protofilaments (95), thus strengthening its MAP resemblance. Whereas FtsZ is present in all prokaryotes studied so far (for a review, see reference 66), ZipA does not seem to occur in gram-positive bacteria and archaebacteria, suggesting that in these cases other proteins might have acquired a similar role (95). Of course, these findings have not yet elucidated how the prokaryotic FtsZ ring evolved into a eukaryotic spindle apparatus for the separation of replicated chromosomes. One clue linking FtsZ to chromosome segregation might be, as mentioned above, that impairment of FtsZ affects the segregation of completed chromosomes in E. coli (45, 113). Another component of the E. coli divisome, FtsA, has recently been crystallized from the thermophilic eubacterium Thermotoga maritima (114). This protein bears a strong resemblance to actin and could therefore be the predecessor of actin in the eukaryotic cytokinetic ring (114). In the eukaryotic cytokinetic system, motor proteins interact with microtubules and actin filaments whereas motor proteins are sparse in prokaryotes. As a speculation, MukB, resembling dynamin, might have been an example for motor proteins to develop. CONCLUSION Cytokinesis requires duplication of cellular structures followed by bipolarization of the predivisional cell. As a common principle, this applies to prokaryotes as well as eukaryotes. However, the macromolecular mechanisms to achieve this are essentially different. As pointed out above, duplication of a eukaryotic origin of DNA replication, as well as its genome, does not suffice to establish a bipolar cell. It requires the duplication of the MTOC as a prelude to mitosis, and it is here that the dynamic cytoskeleton with all its associated proteins comes to the fore. Eukaryotes, because of their size, have thus created a higher level of organization, where the MTOC has taken over the segregating function of the prokaryotic origin of replication. In prokaryotes, a cytoskeleton that pervades the cytoplasm appears to be absent. DNA replication and the concomitant DNA segregation seem to occur without the help of extensive supramacromolecular assemblies. Prokaryotic cytokinesis, however, proceeds through a contracting ring, which has a roughly 100-fold-smaller circumference than its eukaryotic counterpart. Its macromolecular composition is also essentially different, although it contains proteins that can be considered predecessors of actin, tubulin, and MAPs.

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