sign in sign up
<<
Home , Crescentin, Cytoskeleton, ParM

Available online at www.sciencedirect.com

The bacterial Joe Pogliano

Bacteria contain a complex cytoskeleton that is more diverse FtsZ from and form a family of highly than previously thought. Recent research provides insight into conserved that are very divergent from eukar- how bacterial , , and ParA proteins participate in a yotic tubulins, with only amino acids involved in GTP variety of cellular processes. binding and hydrolysis conserved between the two families [5–8]. Despite this divergence the three-dimen- Addresses sional structures of FtsZ and are very similar, Division of Biological Sciences, University of California San Diego, 9500 suggesting they evolved from a common ancestor [5–10]. Gilman Drive, La Jolla, CA 92093-0377, United States Like tubulin, FtsZ polymerizes cooperatively and in a GTP-dependent manner in vitro [7–12]. FtsZ is an essen- Corresponding author: Pogliano, Joe ([email protected]) tial component of the division apparatus, assembling a cytokinetic ring at midcell required to recruit other Current Opinion in Cell 2008, 20:19–27 members of the complex [5,8–10,13–16]. The FtsZ ring constricts with septum invagination and This review comes from a themed issue on Cell structure and dynamics reassembles at new division sites from spirals of FtsZ [17– Edited by Yixian Zheng and Karen Oegema 20]. In addition to recruiting septal biogenesis to the cell midpoint, recent reports implicate FtsZ in affect- ing peptidoglycan synthesis along the side wall as well   0955-0674/$ – see front matter [21 ,22 ]. Published by Elsevier Ltd. In vitro, purified FtsZ assembles protofilaments, tubes DOI 10.1016/j.ceb.2007.12.006 and sheets under a variety of different polymerization conditions, but how FtsZ are arranged in vivo Introduction has been unclear. New techniques such as electron Bacterial cells have a complex subcellular organization that cryotomography that allow high-resolution imaging of  is established and maintained by a diverse set of - cells in a near-native state [23,24,25 ] promise to reveal izing proteins that make up the bacterial cytoskeleton. At the in vivo structure of FtsZ and many other bacterial least three general classes of dynamic polymers have been cytoskeletal filaments. The first high-resolution glimpse identified: proteins with to the eukaryotic poly- of the FtsZ ring of using electron mers and tubulin, and members of the ParA/MinD cryotomography was recently provided by Li et al.  family. Among the bacterial actins, at least five different [26 ]. FtsZ rings were observed to consist of multiple, familieshavebeencharacterizedandshowntoparticipatein short (100 nm) overlapping protofilaments approxi- many processes, including cell division, maintaining cell mately 5-nm wide (Figure 1a). Surprisingly, these shape,positioningbacterialorganelles, and catalyzingDNA filaments always occurred about 16 nm away from segregation. Most known bacterial tubulins are closely the , suggesting the existence of an related and are required for cell division, but recent work adaptor that links the filaments to the mem- has identified additional divergent members that partici- brane. pate in DNA replication or segregation. The ParA/ MinD superfamily of ATPases form a large and diverse set BtubA/BtubB ofproteinsthatrelyupontheirdynamicassemblyproperties At least eight families of tubulin have been described in to mediate the localization of many types of protein com- , while in bacteria the only tubulin relative plexes within the cell and for catalyzing the segregation of recognized for many years was FtsZ. The availability of both plasmid and chromosomal DNA. Several in-depth genomic sequences recently led to the identification of reviews have recently focused on the bacterial cytoskeleton several additional families of tubulin-like proteins [1–4]. This review highlights recent progress on these three encoded within bacterial and archaeal genomes  highly conserved classes of cytoskeletal proteins with an [8,27 ,28–30]. A pair of tubulin homologs, BtubA and emphasis on new insights into how they function and on the BtubB, characterized from Prosthebacter dejoneii were identification of recently discovered family members. shown to be closely related to a and b tubulin and assemble as a heterodimer into GTP-dependent poly- Bacterial tubulins mers in vitro [28–30]. BtubA/BtubB were probably FtsZ acquired from a eukaryotic cell by horizontal trans- One of the first cytoskeletal proteins recognized in bac- fer. The functions of the BtubA/BtubB polymers within teria was the tubulin homolog FtsZ. The sequences of Prosthebacter are currently unknown [28–30]. www.sciencedirect.com Current Opinion in 2008, 20:19–27 20 Cell structure and dynamics

Figure 1

Progress in understanding the bacterial cytoskeleton is revealed in a collection of cell biology images from the last year. (a) AreconstructionofFtsZ protofilaments (red) near the inner membrane (blue) based on electron cryotomography of C. crescentus. The outer membrane is shown in green. The panel on the right shows the localization of FtsZ-GFP at the division site of C. crescentus. Reprinted from [26] with permission from the publisher. (b) TubZ-GFP assembles polymers required to stably maintain plasmid pBtoxis in thuringiensis [27]. (c) Fluorescently labeled ParM (green) polymerizes between two beads (yellow) coated with parC DNA bound with ParR, pushing the beads apart over time (s). The right two panels show electron microscopy images of ParM filaments attached to the beads. Reprinted from [77] with permission from the publisher. (d) A phylogenetic tree showing the relationship of several of the known families of bacterial actins. The bottom panel shows that the B. subtilis plasmid segregation protein AlfA assembles polymers (green) extending throughout the cell (red membranes). FRAP experiments (right two panels) show that AlfA-GFP filaments dynamically exchange subunits. Reprinted from [80] with permission from the publisher. (e) C. crescentus MipZ interacts with ParB at the cell poles and assembles a protein gradient (graph) that prevents FtsZ from assembling near the poles, thereby favoring FtsZ assembly at midcell. Reprinted from [103] with permission from the publisher. (f) V. cholerae ParA1-GFP (red) migrates in front of the separating YFP-ParB-labeled origins (green), suggesting a mitotic mechanism in which ParA pulls the originsapart. Panels I through VI show different cells at various stages of the cell cycle. Reprinted from [114] with permission from the publisher.

TubZ and RepX tubulins identified thus far are encoded by large Many bacteria and archaea encode relatives of tubulin in various species of Bacillus [27]. Recent work demon- and FtsZ that are so vastly divergent that they do not fit strates that some of these proteins comprise a previously into either family [8,27]. All of the divergent bacterial unrecognized tubulin-based bacterial cytoskeleton. The

Current Opinion in Cell Biology 2008, 20:19–27 www.sciencedirect.com The bacterial cytoskeleton Pogliano 21

first member of this family shown to polymerize was gence of the archaeal proteins, they might have alterna- TubZ from Bacillus thuringiensis [27]. TubZ is encoded tive functions, raising the possibility that divergent by pBtoxis, a virulence plasmid that carries several of the tubulin homologs, like divergent bacterial actins, assem- insecticidal crystal toxins for which B. thuringiensis is well ble a variety of different types of polymers that participate known [31]. TubZ-GFP fusions assemble dynamic poly- in many different aspects of cellular physiology. mers in B. thuringiensis that span the length of the cell [27](Figure 1b). In time-lapse microscopy and FRAP Bacterial actins experiments, TubZ-GFP polymers are polarized with MreB plus and minus ends and translocate through the cell Bacteria contain many proteins distantly related to eukar- by a -type mechanism. TubZ can assemble yotic actins. FtsA, MreB, and ParM were long ago recog- by itself in either B. thuringiensis or Escherichia coli, and nized to contain key amino acid motifs conserved within appears to have a critical concentration for assembly in the larger actin/hsp70/hexokinase superfamily [35]. Elu- vivo. cidation of the crystal structure of MreB and the discovery that it assembles filaments in vitro and in vivo demon- TubZ appears to play an important role in stably main- strated that these divergent actins are part of an essential taining plasmid pBtoxis. A mutant TubZ protein bacterial cytoskeleton that probably arose billions of years (TubZD269A) predicted to be defective in GTP hydroly- ago [9,36–39]. MreB and closely related proteins (such as sis assembles static rather than dynamic polymers. When B. sutbilis Mbl and MreBH) assemble dynamic polymers the mutant protein is expressed in trans from a compatible that move rapidly in a tight spiral pattern beneath the cell plasmid, it coassembles with wild-type TubZ, trapping it membrane in many different organisms [37,38,40–44]. in a nonfunctional form, and this leads to loss of pBtoxis The mechanism of movement could be via treadmilling, from the cell [27]. TubZ is encoded in an operon as reported for MreB-YFP in C. crescentus [45]. Purified together with TubR, a DNA-binding protein that MreB from Thermotoga maritima assembles actin-like regulates TubZ expression. It therefore seems likely that polymers in the presence of either GTP or ATP TubZ and TubR are essential components of a plasmid [36,39,46]. maintenance machinery, but their precise roles are still not understood. Given TubZ’s dynamic assembly proper- Proteins of the MreB family have several important ties, one possibility is that TubZ plays a role in plasmid functions, the most conserved being a direct role in DNA segregation, potentially representing a very simple maintaining cell shape by influencing the position of and ancient tubulin-based mitotic apparatus. However, as peptidoglycan synthesis [37,38,42,47–52]. Cell shape con- discussed below, these proteins might be also involved in trol requires the concerted actions of MreB with several DNA replication. other proteins, including MreC, MreD, and Pbp2 [53–59]. Many bacteria contain only a single MreB protein, but in How conserved are the polymerization properties of B. subtilis, depends upon three closely related TubZ? At least four other Bacillus plasmids encode tubu- proteins, MreB, MreBH, and Mbl, all of which colocalize lin-like proteins, each very distantly related to the other within the cell [37,48,60]. MreBH interacts with a cell and to TubZ [27]. One of these, RepX encoded by wall hydrolyase (LytE) and directs its localization in a plasmid pX01 of Bacillus anthracis, was identified as an helical pattern, providing a potential mechanism by important component of plasmid replication [32]. A which MreBH can directly influence peptidoglycan struc- mini-replicon constructed from pX01 could only be intro- ture [60]. duced into B. anthracis by electroporation if an intact copy of RepX was present. A parallel finding was also made for In addition to maintaining cell shape, MreB participates pBtoxis, where a mini-replicon containing TubZ and in a number of other functions within the cell including TubR was constructed [33]. RepX was shown by electron protein localization and segregation microscopy and dynamic light scattering to undergo (reviewed in [1,4]) [41,51,56,61–69]. The extent to which dynamic, GTP-dependent polymerization in vitro [34]. MreB directly functions in chromosome segregation is RepX has a GTPase activity that is required to establish still being investigated in some organisms [66,68], but in the plasmid in vivo during transformation experiments an elegant study of C. crescentus, a direct role for MreB in [32]. Taken together, it is now clear that these divergent chromosomal origin separation was established [67]. tubulins (TubZ and RepX) are important for plasmid Using a small molecule inhibitor (A22) that allowed stability, functioning in replication, segregation, or the rapid inhibition of MreB function in synchronized possibly both. It seems likely that these functions are cell cultures [67], inactivation of MreB prevented segre- conserved among the TubZ-like proteins encoded by gation of GFP-tagged chromosomal origins without plasmids in B. megaterium and B. cereus. A function in affecting DNA replication. Future studies using small DNA stability might also be conserved among the molecules to inhibit MreB and other cytoskeletal proteins archaeal TubZ-like proteins, many of which are also will probably be instrumental in deciphering the many plasmid-encoded. However, given the extreme diver- functions of these dynamic proteins. www.sciencedirect.com Current Opinion in Cell Biology 2008, 20:19–27 22 Cell structure and dynamics

ParM end in the electron microscope, predicts that several ParM is a bacterial actin required for segregation of the E. amino acids on the outer helix should be important for coli plasmid R1 [70–72]. MreB and ParM have three- DNA binding. Mutations in these amino acids eliminate dimensional structures similar to actin, though all three both DNA binding and segregation functions, providing share an incredibly low level of sequence similarity additional support for the model. Another surprising (<12%) [36,73]. ParM assembles polymers that act aspect of the structure is the presence of a small together with the ParR DNA-binding protein and its (6 nm diameter) hole just large enough to accommodate cognate DNA recognition sites (clustered within a cen- a ParM filament if amino acids of the flexible C-terminal tromere-like region of DNA, parC) to segregate plasmid helix of ParR give way. These in vitro studies of ParM and DNA [74,75]. The polymerization dynamics of ParM are ParR suggest a detailed model for the architecture of the very different from actin [76,77]. Purified ParM displays plasmid partition complex in which ParM filaments inter- dynamic instability in vitro in which spontaneously act with the internal C-terminus of the ParR helix bound nucleated filaments extend bidirectionally at equal rates to parC DNA. Two ends of spontaneously nucleated and rapidly decay if not stabilized by interactions with ParM filaments become stabilized by ParR/parC, and as ParR/parC nucleoprotein complexes [76]. Remarkably, ParM-ATP monomers add to both growing ends of a this entire system was reconstituted in vitro with only filament, multiple sites of interaction with the ParR helix these three components [77]. Fluorescently labeled increase processivity, driving plasmids apart without let- ParM assembled a radial array of dynamically unstable ting go. An elegant feature of the ParR helix structure is polymers when added to beads coated with parC DNA its complete symmetry, allowing ParM filaments to inter- and ParR. When both ends of a dynamic polymer became act with plasmids from either side. captured by parC/ParR complexes, polymerization con- tinued by incorporation of new subunits adjacent to the AlfA nucleoprotein complexes, driving the beads rapidly apart Another highly divergent actin relative, AlfA, was [77](Figure 1c). recently shown to play a role in segregating plasmid DNA during both vegetative growth and sporulation in A refined model for ParM filaments recently demon- B. subtilis [80]. The process of sporulation in B. subtilis strated that they have a left-handed twist rather than a poses a unique challenge for plasmid inheritance, because right-handed twist like actin [78]. ParM, like other plasmids must localize to one extreme end of the cell actins, contains a central nucleotide-binding cleft formed before polar septation to be inherited by the spore [80]. by two domains of the protein. The degree of opening Actin-like polymers that assemble between plasmids and between the two domains of a single monomer is pro- push them toward the cell pole would be ideally suited for posed to contribute to the degree of filament twist this function. AlfA assembles polymers that can extend observed in vitro [78]. The model generated suggests the entire length of the cell (Figure 1d). Fluorescence that the subunit–subunit contacts within the ParM fila- recovery after photobleaching (FRAP) experiments ments are different from that of F-actin. These new demonstrate that these polymers are highly dynamic, results lead to the idea that divergent actin relatives rapidly exchanging subunits with a cytoplasmic pool. may assemble a variety of different structures. Given Like many other plasmid segregation systems, a DNA- the significant sequence divergence of many bacterial binding protein, AlfB, is also required for segregation and actins, even within the MreB family, the implication is probably serves as an adaptor protein connecting the that there may in fact be a large number of ways actin-like filament to the DNA. Unlike ParM, dynamic instability proteins have evolved to assemble a polymer. Evidence has yet to be observed for AlfA, raising the question of for this last point awaits the structural characterization of how similar the mechanism of AlfA-mediated segregation additional divergent members of the bacterial actin super- is to that described for ParM. family. MamK How do ParM filaments attach to the plasmid DNA? An Magnetotactic bacteria synthesize unique elegant structure provided by Moller-Jensen [79] now called that they use to align themselves provide a possible mechanism by which ParR may pro- in a geomagnetic field. In Magnetospirillum species, mag- vide a link to the filament. The DNA binding N-terminus netosome formation depends upon a cytoskeletal network of ParR forms a ribbon–helix–helix structure that binds composed at least in part of MamJ and MamK cooperatively to 10 sites within parC [75,79]. Surpris- [81,82,83,84,85]. As visualized by electron cryoto- ingly, ParR dimers assemble into a gently curved helix mography, the actin homolog MamK assembles a series of with 12 dimers required to make one full turn. The DNA 6-nm wide linear filaments that extend over much of the recognition domains are positioned with a spacing of length of the cell and provide a scaffold for the assembly 3.5 nm on the outside of the ParR helix to make site- of membrane invaginations containing magnetite (mag- specific contacts as the DNA wraps around. This helical netosomes). MamJ is thought to be required to attach structure, which appears as a large donut when viewed on magnetosomes to the polymer [84]. MamJ and MamK

Current Opinion in Cell Biology 2008, 20:19–27 www.sciencedirect.com The bacterial cytoskeleton Pogliano 23

directly interact and mutations in the encoding C. crescentus MipZ either protein disrupt the linear arrangement of magneto- An elegant strategy for determining the position of FtsZ somes [81,83,84]. Purified MamK assembles into assembly independently of the Min system was recently bundles of filaments in vitro [85]. It will be interesting elucidated in C. crescentus, where the ParA family mem- to see a comparison of the biochemical and structural ber MipZ was shown to couple chromosome segregation properties of MamK with ParM, MreB, and AlfA. to cell division [103]. At the beginning of the cell cycle, MipZ localizes to the chromosomal origin of FtsA replication (oriC) via the DNA-binding protein ParB. Sequence and structural data show that the cell division After DNA replication, one of the daughter origins protein FtsA forms a separate and divergent family of migrates to the opposite cell pole simultaneously with bacterial actins [35,86]. FtsA localizes to the future site MipZ, which is also an inhibitor of FtsZ polymerization. of cell division by interacting with the C-terminus of The arrival of MipZ and oriC at the opposite pole FtsZ and contributes to the recruitment of other mem- stimulates the release of a pool of FtsZ from this bers of the cell division complex [10,13–15]. For many location, thereby coupling FtsZ assembly to DNA seg- other bacterial actins such as ParM, MreB, and AlfA, the regation. MipZ localizes to both poles after oriC separ- ability to assemble dynamic polymers is central to their ation and establishes a protein gradient within the cell function, but for FtsA, the role of polymerization, if any, that extends inward from each pole (Figure 1e), provid- is currently unclear. Recent studies have shown that S. ing a mechanism for directing the assembly of FtsZ in pneumoniae FtsA assembles polymers in vitro [87], the center of the cell. suggesting that polymerization might also play an important role in FtsA function, but this function Plasmid DNA segregation by ParA remains poorly defined. ParA ATPases are important for the efficient segregation of both plasmid and chromosomal DNA in many bac- Dynamic localization of proteins belonging to teria [3,71]. ParA proteins usually occur in an operon the ParA/MinD superfamily together with a DNA-binding protein, ParB, which The ParA/MinD family of P-loop ATPases [88] are highly interacts with a set of specific DNA-binding sites that conserved in bacteria and are key components of the form the equivalent of a simple centromere parC [3,71]. bacterial cytoskeleton. Two different subfamilies, MinD Plasmids with ParA partitioning systems such as F, P1, and ParA, play a multitude of roles in bacterial subcellular and RK2, are positioned in the middle of the cell, where organization. MinD and related proteins were recognized they remain as the cell continues to grow. After replica- long ago as spatial regulators of the site of cell division, tion, daughter plasmids are separated slightly and gen- but only more recently were they found to assemble erate two distinct plasmid complexes that later rapidly polymers that oscillate rapidly within the cell. ParA separate from each other at approximately 0.2 mm/min. proteins form dynamic polymers that catalyze the segre- During the process of separation, daughter plasmids are gation of plasmid and chromosomal DNA, and more repositioned to the quarter-cell positions, that is, to what recently have been shown to determine the position of will be the midcell positions of the future daughter other protein complexes within the cell. cells.

MinD The ATPase activities of ParA proteins are presumed to In E. coli, MinD works together with MinC and MinE play a central role in targeting plasmids to midcell and in to determine the position of cell division by specifying driving plasmid separation, but the underlying mechan- the assembly of FtsZ at the cell midpoint. MinD is a isms are still unclear. Recent studies from several differ- polymerizing ATPase [89–91] that associates with the ent systems show that plasmid ParA proteins assemble membrane via its C-terminus [92–94] and also with polymers in vivo and in vitro [104,105,106,107,108]. In MinC to form a complex (MinC/MinD) that inhibits many instances, ParA-GFP fusions oscillate within the FtsZ polymerization. MinC and MinD cycle from pole cell and the cognate ParB/parC is required for this to pole along the cell membrane, generating a temporal dynamic localization [104,108–112]. These results have protein gradient in which the highest time-averaged led to a model in which ParA proteins assemble poly- concentration occurs near the cell pole thereby prevent- mers that cycle back and forth both between plasmids ing polar FtsZ assembly [95–98]. Assembly of FtsZ over and on either side of them, such that when the forces the is inhibited by SlmA in E. coli [99] and generated by the polymers are balanced in all directions, Noc in B. subtilis [100]. Although MinC and MinD are a single plasmid is held near the cell midpoint, while conserved in many bacteria, surprisingly, oscillation is two plasmids are held near the quarter cell positions not. In B. subtilis, MinC and MinD localize statically to [107,108]. This model is strikingly different from the the poles by interactions with DivIVA [101,102], mak- ParM model, in which a single, linear filamentous struc- ing it unclear whether dynamic polymerization is an ture (or bundles of filaments) mediates plasmid separ- evolutionarily conserved feature of MinD function. ation [74–76,77]. www.sciencedirect.com Current Opinion in Cell Biology 2008, 20:19–27 24 Cell structure and dynamics

Chromosome segregation by ParA proteins Using electron cryotomography, they discovered at least A number of bacteria contain ParA proteins with a role in four distinct types of filaments within C. crescentus that chromosome segregation [113]. One recent example is could be grouped based on their size and location within cholerae, whose genome is divided into two the cell. Surprisingly, most of these filaments were still (large and small) that replicate and segre- present when known cytoskeletal proteins such as MreB gate separately from each other. Both chromosomes rely and were inactivated, indicating that many more upon proteins related to ParA for efficient segregation families of bacterial cytoskeletal proteins await discovery [114,115,116]. ParA1 from V. cholerae mediates the [25]. segregation of the origin of replication region of the large chromosome from one cell pole to the other [114]. Cells An emerging paradigm in bacterial cell biology is that depleted for ParA1 display chromosome I segregation divergent actin relatives evolved in many different ways defects. ParA1-GFP assembles into a cytoplasmic haze to harness the mechanical properties of polymerization for of fluorescence that, in time-lapse microscopy exper- a variety of different processes, including cell division, cell iments, migrates in front of the moving origin of replica- shape, and DNA segregation. This theme likely applies to tion (Figure 1f). These results suggest a model for other families of dynamic polymers, including members of chromosome segregation in which ParA1 assembles a the ParA/MinD family that use dynamic localization to mitotic machinery that pulls the daughter origins apart position protein and DNA molecules within the cell, and [114]. The smaller chromosome relies upon its own possibly also applies to divergent bacterial tubulins, which ParA system (ParA2) to position the origin of replication assemble dynamic cytoskeletal polymers required for plas- near the mid- and quarter cell positions [115]. mid maintenance. Our knowledge of cytoskeletal proteins will continue to expand with the development of more ParA and the localization of cytoplasmic protein powerful fluorescence and electron microscopy tools. complexes Members of the ParA family have probably evolved to Acknowledgement participate in localizing many different types of protein This work was supported by a grant from the NIH (R01GM073898).  complexes in bacteria [117,118 ]. In R. sphaeroides, for References and recommended reading example, complexes of chemotaxis proteins localize to Papers of particular interest, published within the annual period of the midcell and quarter cell positions [118]. A ParA review, have been highlighted as: homolog (PpfA) encoded within the chemotaxis operon  of special interest was found to be responsible for positioning these che-  of outstanding interest motaxis complexes. In parA mutants, the complexes are mislocalized and the cells are nonmotile, demonstrating 1. Carballido-Lopez R: The bacterial actin-like cytoskeleton. that positioning by PpfA is essential for their function. Microbiol Mol Biol Rev 2006, 70:888-909. The similarities between the localization behavior of 2. Shih YL, Rothfield L: The bacterial cytoskeleton. Microbiol Mol bacterial plasmids and the chemotaxis proteins suggest Biol Rev 2006, 70:729-754. a common positioning mechanism is at work. 3. Hayes F, Barilla D: The bacterial segrosome: a dynamic nucleoprotein machine for DNA trafficking and segregation. Nat Rev Microbiol 2006, 4:133-143. Crescentin 4. Graumann PL: Cytoskeletal elements in bacteria. Crescentin is an intermediate filament protein identified Annu Rev Microbiol 2007, 61:589-618. in C. crescentus in a screen for cell shape mutants [119]. 5. Nogales E, Downing KH,Amos LA, Lowe J: Tubulin and FtsZ form a Crescentin assembles a slightly curved filamentous struc- distinct family of GTPases. Nat Struct Biol 1998, 5:451-458. ture that contributes to Caulobacter’s characteristic comma 6. Nogales E, Wolf SG, Downing KH: Structure of the alpha beta shape appearance. In the absence of crescentin, cells tubulin dimer by electron crystallography. Nature 1998, become rod-shaped but are otherwise viable. The dis- 391:199-203. pensability of intermediate filaments for Caulobacter 7. Lowe J, Amos LA: Crystal structure of the bacterial cell-division protein FtsZ. Nature 1998, 391:203-206. growth and their general absence from many bacteria 8. Vaughan S, Wickstead B, Gull K, Addinall SG: Molecular suggests that it serves a specialized role in these organ- of FtsZ protein sequences encoded within the isms, perhaps contributing to formation of a cell shape genomes of archaea, bacteria, and eukaryota. J Mol Evol 2004, that provides a selective advantage as Caulobacter propels 58:19-29. itself through aqueous environments. 9. van den Ent F, Amos L, Lowe J: Bacterial ancestry of actin and tubulin. Curr Opin Microbiol 2001, 4:634-638. Conclusions 10. Errington J: Dynamic proteins and a cytoskeleton in bacteria. Nat Cell Biol 2003, 5:175-178. The recent identification of many new families of bacterial 2+ polymers (MreB, ParM, MamK, AlfA, TubZ, ParA, and 11. Lowe J, Amos LA: Tubulin-like protofilaments in Ca -induced FtsZ sheets. EMBO J 1999, 18:2364-2371. crescentin) suggests that the bacterial cytoskeleton is more 12. Chen Y, Bjornson K, Redick SD, Erickson HP: A rapid diverse and complex than previously thought. This point fluorescence assay for FtsZ assembly indicates cooperative  was made beautifully most recently by Briegel et al. [25 ]. assembly with a dimer nucleus. Biophys J 2005, 88:505-514.

Current Opinion in Cell Biology 2008, 20:19–27 www.sciencedirect.com The bacterial cytoskeleton Pogliano 25

13. Goehring NW, Beckwith J: Diverse paths to midcell: assembly 30. Schlieper D, Oliva MA, Andreu JM, Lowe J: Structure of bacterial of the bacterial cell division machinery. Curr Biol 2005, tubulin BtubA/B: evidence for horizontal gene transfer. 15:R514-R526. Proc Natl Acad Sci U S A 2005, 102:9170-9175. 14. Margolin W: FtsZ and the division of prokaryotic cells and 31. Berry C, O’Neil S, Ben-Dov E, Jones AF, Murphy L, Quail MA, organelles. Nat Rev Mol Cell Biol 2005, 6:862-871. Holden MT, Harris D, Zaritsky A, Parkhill J: Complete sequence and organization of pBtoxis, the toxin-coding plasmid of 15. Dajkovic A, Lutkenhaus J: Z ring as executor of bacterial cell Bacillus thuringiensis subsp. israelensis. Appl Environ division. J Mol Microbiol Biotechnol 2006, 11:140-151. Microbiol 2002, 68:5082-5095. 16. Romberg L, Levin PA: Assembly dynamics of the bacterial cell 32. Tinsley E, Khan SA: A novel FtsZ-like protein is involved in division protein FTSZ: poised at the edge of stability.  replication of the anthrax toxin-encoding pXO1 plasmid in Annu Rev Microbiol 2003, 57:125-154. Bacillus anthracis. J Bacteriol 2006, 188:2829-2835. This paper demonstrates that a distant tubulin relative, RepX, is required 17. Peters PC, Migocki MD, Thoni C, Harry EJ: A new assembly to construct a mini-pXO1 plasmid, suggesting that RepX is required for pathway for the cytokinetic Z ring from a dynamic helical pX01 replication. structure in vegetatively growing cells of Bacillus subtilis. Mol Microbiol 2007, 64:487-499. 33. Tang M, Bideshi DK, Park HW, Federici BA: Minireplicon from pBtoxis of Bacillus thuringiensis subsp. israelensis. 18. Thanedar S, Margolin W: FtsZ exhibits rapid movement and Appl Environ Microbiol 2006, 72:6948-6954. oscillation waves in helix-like patterns in Escherichia coli. Curr Biol 2004, 14:1167-1173. 34. Anand SP, Akhtar P, Tinsley E, Watkins SC, Khan SA: GTP- dependent polymerization of the tubulin-like RepX replication 19. Ben-Yehuda S, Losick R: Asymmetric cell division in B. subtilis protein encoded by the pXO1 plasmid of Bacillus anthracis. involves a spiral-like intermediate of the cytokinetic protein Mol. Microbiol. 2008, 67:881-890. FtsZ. Cell 2002, 109:257-266. 35. Bork P, Sander C, Valencia A: An ATPase domain common to 20. Grantcharova N, Lustig U, Flardh K: Dynamics of FtsZ assembly prokaryotic cell cycle proteins, sugar kinases, actin, and during sporulation in Streptomyces coelicolor A3(2). J Bacteriol hsp70 heat shock proteins. Proc Natl Acad Sci U S A 1992, 2005, 187:3227-3237. 89:7290-7294. 21. Aaron M, Charbon G, Lam H, Schwarz H, Vollmer W, Jacobs- 36. van den Ent F, Amos L, Lowe J: Prokaryotic origin of the actin  Wagner C: The tubulin homologue FtsZ contributes to cell cytoskeleton. Nature 2001, 413:39-44. elongation by guiding precursor synthesis in 37. Jones L, Carballido-Lopez R, Errington J: Control of cell shape in Caulobacter crescentus. Mol Microbiol 2007, 64:938-952. bacteria: helical, actin-like filaments in Bacillus subtilis. This paper shows that C. crescentus FtsZ is required for medial localiza- Cell 2001, 104:913-922. tion of MurG, a cell wall precursor (lipid II) biosynthetic , and thereby contributes to controlling the spatial positioning of peptidoglycan 38. Carballido-Lopez R, Errington J: The bacterial cytoskeleton: in synthesis during cell elongation. vivo dynamics of the actin-like protein Mbl of Bacillus subtilis. Dev Cell 2003, 4:19-28. 22. Varma A, de Pedro MA, Young KD: FtsZ directs a second mode  of peptidoglycan synthesis in Escherichia coli. J Bacteriol 2007, 39. Esue O, Cordero M, Wirtz D, Tseng Y: The assembly of MreB, a 189:5692-5704. prokaryotic homolog of actin. J Biol Chem 2005, 280:2628- This paper presents evidence for the intriguing idea that E. coli FtsZ 2635. affects side wall synthesis of peptidoglycan near the cell poles in certain mutants. 40. Defeu Soufo HJ, Graumann PL: Dynamic movement of actin-like proteins within bacterial cells. EMBO Rep 2004, 5:789-794. 23. McIntosh JR: Electron microscopy of cells: a new beginning for a new century. J Cell Biol 2001, 153:F25-F32. 41. Gitai Z, Dye N, Shapiro L: An actin-like gene can determine in bacteria. Proc Natl Acad Sci U S A 2004, 101:8643- 24. McIntosh R, Nicastro D, Mastronarde D: New views of cells in 3D: 8648. an introduction to electron tomography. Trends Cell Biol 2005, 15:43-51. 42. Figge RM, Divakaruni AV, Gober JW: MreB, the cell shape- determining bacterial actin homologue, co-ordinates cell wall 25. Briegel A, Dias DP, Li Z, Jensen RB, Frangakis AS, Jensen GJ: in Caulobacter crescentus. Mol Microbiol 2004,  Multiple large filament bundles observed in Caulobacter 51:1321-1332. crescentus by electron cryotomography. Mol Microbiol 2006, 62:5-14. 43. Slovak PM, Wadhams GH, Armitage JP: Localization of MreB in This paper presents the discovery of several types of cytoskeletal struc- Rhodobacter sphaeroides under conditions causing changes in tures in Caulobacter crescentus that do not correspond to known pro- cell shape and membrane structure. J Bacteriol 2005, 187:54-64. teins, suggesting that additional classes of bacterial cytoskeletal proteins 44. Slovak PM, Porter SL, Armitage JP: Differential localization of await discovery. Mre proteins with PBP2 in Rhodobacter sphaeroides. J Bacteriol 2006, 188:1691-1700. 26. Li Z, Trimble MJ, Brun YV, Jensen GJ: The structure of FtsZ  filaments in vivo suggests a force-generating role in cell 45. Kim SY, Gitai Z, Kinkhabwala A, Shapiro L, Moerner WE: division. EMBO J 2007, 26:4694-4708. Single molecules of the bacterial actin MreB undergo directed The paper uses electron cryotomography to determine the arrangement treadmilling motion in Caulobacter crescentus. Proc Natl Acad of FtsZ protofilaments at the site of cell division. Sci U S A 2006, 103:10929-10934. 27. Larsen RA, Cusumano C, Fujioka A, Lim-Fong G, Patterson P, 46. Esue O, Wirtz D, Tseng Y: GTPase activity, structure, and  Pogliano J: Treadmilling of a prokaryotic tubulin-like protein,  mechanical properties of filaments assembled from bacterial TubZ, required for plasmid stability in Bacillus thuringiensis. cytoskeleton protein MreB. J Bacteriol 2006, 188:968-976. Genes Dev 2007, 21:1340-1352. This paper compares the biochemical properties of MreB filaments This paper describes the identification of TubZ, a new type of tubulin- assembled in the presence of various nucleotides and finds that both based cytoskeleton required for plasmid stability in Bacillus. GTP and ATP work equally well. 28. Jenkins C, Samudrala R, Anderson I, Hedlund BP, Petroni G, 47. Doi M, Wachi M, Ishino F, Tomioka S, Ito M, Sakagami Y, Suzuki A, Michailova N, Pinel N, Overbeek R, Rosati G, Staley JT: Genes for Matsuhashi M: Determinations of the DNA sequence of the the cytoskeletal protein tubulin in the bacterial genus mreB gene and of the gene products of the mre region that Prosthecobacter. Proc Natl Acad Sci U S A 2002, 99:17049- function in formation of the rod shape of Escherichia coli cells. 17054. J Bacteriol 1988, 170:4619-4624. 29. Sontag CA, Staley JT, Erickson HP: In vitro assembly and 48. Daniel RA, Errington J: Control of cell morphogenesis in GTPase hydrolysis by bacterial tubulins BtubA and BtubB. bacteria: two distinct ways to make a rod-shaped cell. J Cell Biol 2005, 169:233-238. Cell 2003, 113:767-776. www.sciencedirect.com Current Opinion in Cell Biology 2008, 20:19–27 26 Cell structure and dynamics

49. Hu B, Yang G, Zhao W, Zhang Y, Zhao J: MreB is important for sphere after inhibition of Escherichia coli MreB by A22. cell shape but not for chromosome segregation of the Mol Microbiol 2007, 65:51-63. filamentous cyanobacterium Anabaena sp. PCC 7120. Mol Microbiol 2007, 63:1640-1652. 67. Gitai Z, Dye NA, Reisenauer A, Wachi M, Shapiro L: MreB actin- mediated segregation of a specific region of a bacterial 50. Scheffers DJ, Jones LJ, Errington J: Several distinct localization chromosome. Cell 2005, 120:329-341. patterns for penicillin-binding proteins in Bacillus subtilis. Mol Microbiol 2004, 51:749-764. 68. Formstone A, Errington J: A magnesium-dependent mreB null mutant: implications for the role of mreB in Bacillus subtilis. 51. Shih YL, Kawagishi I, Rothfield L: The MreB and Min Mol Microbiol 2005, 55:1646-1657. cytoskeletal-like systems play independent roles in prokaryotic polar differentiation. Mol Microbiol 2005, 69. Nilsen T, Yan AW, Gale G, Goldberg MB: Presence of multiple 58:917-928. sites containing polar material in spherical Escherichia coli cells that lack MreB. J Bacteriol 2005, 187:6187-6196. 52. Mohammadi T, Karczmarek A, Crouvoisier M, Bouhss A, Mengin-Lecreulx D, den Blaauwen T: The essential 70. Jensen RB, Gerdes K: Partitioning of plasmid R1. The ParM peptidoglycan glycosyltransferase MurG forms a complex protein exhibits ATPase activity and interacts with the with proteins involved in lateral envelope growth as well as centromere-like ParR–parC complex. J Mol Biol 1997, with proteins involved in cell division in Escherichia coli. Mol 269:505-513. Microbiol 2007, 65:1106-1121. 71. Jensen R, Lurz R, Gerdes K: Mechanism of DNA segregation 53. Divakaruni AV, Loo RR, Xie Y, Loo JA, Gober JW: The cell-shape in : replicon pairing by parC of plasmid R1. protein MreC interacts with extracytoplasmic proteins Proc Natl Acad Sci U S A 1998, 95:8550-8555. including cell wall assembly complexes in Caulobacter crescentus. Proc Natl Acad Sci U S A 2005, 102:18602-18607. 72. Gerdes K, Moller-Jensen J, Ebersbach G, Kruse T, Nordstrom K: Bacterial mitotic machineries. Cell 2004, 116:359-366. 54. Dye NA, Pincus Z, Theriot JA, Shapiro L, Gitai Z: Two independent spiral structures control cell shape in Caulobacter. 73. van den Ent F, Moller-Jensen J, Amos LA, Gerdes K, Lowe J: Proc Natl Acad Sci U S A 2005, 102:18608-18613. F-actin-like filaments formed by plasmid segregation protein ParM. EMBO J 2002, 21:6935-6943. 55. Leaver M, Errington J: Roles for MreC and MreD proteins in helical growth of the cylindrical cell wall in Bacillus subtilis. 74. Moller-Jensen J, Jensen R, Lowe JKG: Prokaryotic DNA Mol Microbiol 2005, 57:1196-1209. segregation by an actin-like filament. EMBO J 2002, 21:3119-3127. 56. Defeu Soufo HJ, Graumann PL: Bacillus subtilis actin-like protein MreB influences the positioning of the replication 75. Moller-Jensen J, Borch J, Dam M, Jensen RB, Roepstorff P, machinery and requires membrane proteins MreC/D and other Gerdes K: Bacterial : ParM of plasmid R1 moves actin-like proteins for proper localization. BMC Cell Biol 2005, plasmid DNA by an actin-like insertional polymerization 6:10. mechanism. Mol Cell 2003, 12:1477-1487. 57. Kruse T, Bork-Jensen J, Gerdes K: The morphogenetic MreBCD 76. Garner EC, Campbell CS, Mullins RD: Dynamic instability in a proteins of Escherichia coli form an essential membrane- DNA-segregating prokaryotic actin homolog. Science 2004, bound complex. Mol Microbiol 2005, 55:78-89. 306:1021-1025. 58. van den Ent F, Leaver M, Bendezu F, Errington J, de Boer P, 77. Garner EC, Campbell CS, Weibel DB, Mullins RD: Reconstitution Lowe J: Dimeric structure of the cell shape protein MreC and  of DNA segregation driven by assembly of a prokaryotic actin its functional implications. Mol Microbiol 2006, 62:1631-1642. homolog. Science 2007, 315:1270-1274. This landmark paper describes the complete reconstitution of DNA 59. Divakaruni AV, Baida C, White CL, Gober JW: The cell shape segregation from purified components and shows that ParM filaments proteins MreB and MreC control cell morphogenesis by can push fluorescent beads apart. positioning cell wall synthetic complexes. Mol Microbiol 2007, 66:174-188. 78. Orlova A, Garner EC, Galkin VE, Heuser J, Mullins RD,  Egelman EH: The structure of bacterial ParM filaments. Nat 60. Carballido-Lopez R, Formstone A, Li Y, Ehrlich SD, Noirot P, Struct Mol Biol 2007, 14:921-926.  Errington J: Actin homolog MreBH governs cell morphogenesis This paper provides a refined structure for ParM filaments and demon- by localization of the cell wall hydrolase LytE. Dev Cell 2006, strates that they are left-handed rather than right-handed like actin. 11:399-409. This paper shows that MreBH colocalizes with MreB and Mbl and that it 79. Moller-Jensen J, Ringgaard S, Mercogliano CP, Gerdes K, Lowe J: interacts with LytE, an extracellular endopeptidase. An intriguing model is  Structural analysis of the ParR/parC plasmid partition suggested in which MreBH determines the helical localization pattern of complex. EMBO J 2007, 26:4413-4422. LytE by directing the location of its export to the periplasm. The structure of the ParR/parC plasmid DNA complex provides insight into the mechanism by which ParM filaments attach to the DNA. 61. Soufo HJ, Graumann PL: Actin-like proteins MreB and Mbl from Bacillus subtilis are required for bipolar positioning of 80. Becker E, Herrera NC, Gunderson FQ, Derman AI, Dance AL, replication origins. Curr Biol 2003, 13:1916-1920.  Sims J, Larsen RA, Pogliano J: DNA segregation by the bacterial actin AlfA during Bacillus subtilis growth and development. 62. Kruse T, Moller-Jensen J, Lobner-Olesen A, Gerdes K: EMBO J 2006, 25:5919-5931. Dysfunctional MreB inhibits chromosome segregation in This paper describes a new family of dynamic bacterial actins (AlfA) Escherichia coli. EMBO J 2003, 22:5283-5292. involved in plasmid segregation and demonstrates that Bacillus plasmids enter the spore very early in the spore formation pathway. 63. Kruse T, Blagoev B, Lobner-Olesen A, Wachi M, Sasaki K, Iwai N, Mann M, Gerdes K: Actin homolog MreB and RNA polymerase 81. Komeili A, Li Z, Newman DK, Jensen GJ: Magnetosomes are cell interact and are both required for chromosome segregation in  membrane invaginations organized by the actin-like protein Escherichia coli. Genes Dev 2006, 20:113-124. MamK. Science 2006, 311:242-245. This paper, along with 83, demonstrates that organization 64. Kruse T, Gerdes K: Bacterial DNA segregation by the actin-like relies upon an actin-like cytoskeletal protein. MreB protein. Trends Cell Biol 2005, 15:343-345. 82. Pradel N, Santini CL, Bernadac A, Fukumori Y, Wu LF: Biogenesis 65. Srivastava P, Demarre G, Karpova TS, McNally J, Chattoraj DK: of actin-like bacterial cytoskeletal filaments destined for Changes in nucleoid morphology and origin localization upon positioning prokaryotic magnetic organelles. Proc Natl Acad inhibition or alteration of the actin homolog, MreB, of Vibrio Sci U S A 2006, 103:17485-17489. cholerae. J Bacteriol 2007, 189:7450-7463. 83. Scheffel A, Schuler D: The acidic repetitive domain of the 66. Karczmarek A, Martinez-Arteaga R, Alexeeva S, Hansen FG,  Magnetospirillum gryphiswaldense MamJ protein displays Vicente M, Nanninga N, den Blaauwen T: DNA and origin region hypervariability but is not required for magnetosome chain segregation are not affected by the transition from rod to assembly. J Bacteriol 2007, 189:6437-6446.

Current Opinion in Cell Biology 2008, 20:19–27 www.sciencedirect.com The bacterial cytoskeleton Pogliano 27

This paper demonstrates that MamJ and MamK directly interact, provid- 103. Thanbichler M, Shapiro L: MipZ, a spatial regulator coordinating ing additional evidence for a model in which magnetosomes are anchored  chromosome segregation with cell division in Caulobacter. to the MamK filament. Cell 2006, 126:147-162. This landmark paper describes how Caulobacter cells coordinate the 84. Scheffel A, Gruska M, Faivre D, Linaroudis A, Plitzko JM, assembly of FtsZ at midcell with chromosome segregation.  Schuler D: An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria. Nature 2006, 104. Hatano T, Yamaichi Y, Niki H: Oscillating focus of SopA 440:110-114. associated with filamentous structure guides partitioning of F This paper, along with 80, demonstrates that magnetosomes require a plasmid. Mol Microbiol 2007, 64:1198-1213. cytoskeleton for organization. 85. Taoka A, Asada R, Wu LF, Fukumori Y: The polymerization of 105. Barilla D, Rosenberg MF, Nobbmann U, Hayes F: Bacterial DNA actin-like protein MamK associated with magnetosomes. segregation dynamics mediated by the polymerizing protein J Bacteriol 2007, 189:8737-8740. ParF. EMBO J 2005, 24:1453-1464. 86. van den Ent F, Lowe J: Crystal structure of the cell division 106. Bouet JY, Ah-Seng Y, Benmeradi N, Lane D: Polymerization of protein FtsA from Thermotoga maritima. EMBO J 2000,  SopA partition ATPase: regulation by DNA binding and SopB. 19:5300-5307. Mol Microbiol 2007, 63:468-481. This paper demonstrates that SopA polymerization is regulated by DNA 87. Lara B, Rico AI, Petruzzelli S, Santona A, Dumas J, Biton J, and the SopB DNA-binding protein, providing mechanistic insight into Vicente M, Mingorance J, Massidda O: Cell division in cocci: how these polymers may be regulated within the cell. localization and properties of the Streptococcus pneumoniae FtsA protein. Mol Microbiol 2005, 55:699-711. 107. Ebersbach G, Ringgaard S, Moller-Jensen J, Wang Q, Sherratt DJ, Gerdes K: Regular cellular distribution of plasmids by 88. Koonin EV: A superfamily of ATPases with diverse functions oscillating and filament-forming ParA ATPase of plasmid containing either classical or deviant ATP-binding motif. pB171. Mol Microbiol 2006, 61:1428-1442. J Mol Biol 1993, 229:1165-1174. 89. de Boer PA, Crossley RE, Hand AR, Rothfield LI: The MinD protein 108. Lim GE, Derman AI, Pogliano J: Bacterial DNA segregation by is a membrane ATPase required for the correct placement of dynamic SopA polymers. Proc Natl Acad Sci U S A 2005, the Escherichia coli division site. EMBO J 1991, 10:4371-4380. 102:17658-17663. 90. Shih YL, Le T, Rothfield L: Division site selection in Escherichia 109. Marston AL, Errington J: Dynamic movement of the ParA-like coli involves dynamic redistribution of Min proteins within Soj protein of B. subtilis and its dual role in nucleoid coiled structures that extend between the two cell poles. organization and developmental regulation. Mol Cell 1999, Proc Natl Acad Sci U S A 2003, 100:7865-7870. 4:673-683. 91. Suefuji K, Valluzzi R, RayChaudhuri D: Dynamic assembly of 110. Quisel JD, Lin DC, Grossman AD: Control of development by MinD into filament bundles modulated by ATP, phospholipids, altered localization of a transcription factor in B. subtilis. and MinE. Proc Natl Acad Sci U S A 2002, 99:16776-16781. Mol Cell 1999, 4:665-672. 92. Szeto TH, Rowland SL, Rothfield LI, King GF: Membrane 111. Ebersbach G, Gerdes K: The double par locus of virulence localization of MinD is mediated by a C-terminal motif that is factor pB171: DNA segregation is correlated with oscillation of conserved across eubacteria, archaea, and . ParA. Proc Natl Acad Sci U S A 2001, 98:15078-15083. Proc Natl Acad Sci U S A 2002, 99:15693-15698. 112. Ebersbach G, Gerdes K: Bacterial mitosis: partitioning 93. Hu Z, Lutkenhaus J: A conserved sequence at the C-terminus of protein ParA oscillates in spiral-shaped structures and MinD is required for binding to the membrane and targeting positions plasmids at mid-cell. Mol Microbiol 2004, 52: MinC to the septum. Mol Microbiol 2003, 47:345-355. 385-398. 94. Lackner LL, Raskin DM, de Boer PA: ATP-dependent 113. Draper GC, Gober JW: Bacterial chromosome segregation. interactions between Escherichia coli Min proteins and the Annu Rev Microbiol 2002, 56:567-597. phospholipid membrane in vitro. J Bacteriol 2003, 185:735-749. 114. Fogel MA, Waldor MK: A dynamic, mitotic-like mechanism for 95. Hu Z, Lutkenhaus J: Topological regulation of cell division in  bacterial chromosome segregation. Genes Dev 2006, Escherichia coli involves rapid pole to pole oscillation of the 20:3269-3282. division inhibitor MinC under the control of MinD and MinE. This paper describes a model for segregation of the V. cholerae large Mol Microbiol 1999, 34:82-90. chromosome in which ParA1 assembles a mitotic apparatus that pulls the 96. Raskin DM, de Boer PA: MinDE-dependent pole-to-pole daughter origins apart. oscillation of division inhibitor MinC in Escherichia coli. J Bacteriol 1999, 181:6419-6424. 115. Yamaichi Y, Fogel MA, Waldor MK: par genes and the pathology  of chromosome loss in Vibrio cholerae. Proc Natl Acad Sci U S A 97. Raskin DM, de Boer PA: Rapid pole-to-pole oscillation of a 2007, 104:630-635. protein required for directing division to the middle of This paper demonstrates that ParA2, a homolog of plasmid partitioning Escherichia coli. Proc Natl Acad Sci U S A 1999, 96:4971-4976. genes, is essential for segregation of chromosome II in V. cholerae. 98. Hu Z, Lutkenhaus J: Topological regulation of cell division in 116. Saint-Dic D, Frushour BP, Kehrl JH, Kahng LS: A parA homolog E. coli spatiotemporal oscillation of MinD requires stimulation selectively influences positioning of the large chromosome of its ATPase by MinE and phospholipid. Mol Cell 2001, origin in Vibrio cholerae. J Bacteriol 2006, 188:5626-5631. 7:1337-1343. 117. Atmakuri K, Cascales E, Burton OT, Banta LM, Christie PJ: 99. Bernhardt TG, de Boer PA: SlmA, a nucleoid-associated, FtsZ Agrobacterium ParA/MinD-like VirC1 spatially coordinates binding protein required for blocking septal ring assembly early conjugative DNA transfer reactions. EMBO J 2007, over chromosomes in E. coli. Mol Cell 2005, 18:555-564. 26:2540-2551. 100. Wu LJ, Errington J: Coordination of cell division and chromosome segregation by a nucleoid occlusion protein in 118. Thompson SR, Wadhams GH, Armitage JP: The positioning of Bacillus subtilis. Cell 2004, 117:915-925.  cytoplasmic protein clusters in bacteria. Proc Natl Acad Sci U S A 2006, 103:8209-8214. 101. Marston AL, Thomaides HB, Edwards DH, Sharpe ME, Errington J: A chromosomally encoded ParA protein is shown to position chemotaxis Polar localization of the MinD protein of Bacillus subtilis and protein complexes, demonstrating the generally important role of these its role in selection of the mid-cell division site. Genes Dev proteins in establishing and maintaining bacterial subcellular organiza- 1998, 12:3419-3430. tion. 102. Marston AL, Errington J: Selection of the midcell division site in 119. Ausmees N, Kuhn JR, Jacobs-Wagner C: The bacterial Bacillus subtilis through MinD-dependent polar localization cytoskeleton: an intermediate filament-like function in cell and activation of MinC. Mol Microbiol 1999, 33:84-96. shape. Cell 2003, 115:705-713. www.sciencedirect.com Current Opinion in Cell Biology 2008, 20:19–27