Available online at www.sciencedirect.com The bacterial cytoskeleton Joe Pogliano Bacteria contain a complex cytoskeleton that is more diverse FtsZ from bacteria and archaea form a family of highly than previously thought. Recent research provides insight into conserved proteins that are very divergent from eukar- how bacterial actins, tubulins, 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 tubulin 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 cell division apparatus, assembling a cytokinetic ring at midcell required to recruit other Current Opinion in Cell Biology 2008, 20:19–27 members of the cell division 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 enzymes 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 polymers 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 polymer- 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 homology to the eukaryotic poly- of the FtsZ ring of Caulobacter crescentus using electron mers actin 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 cell membrane, suggesting the existence of an related and are required for cell division, but recent work adaptor protein that links the filaments to the mem- has identified additional divergent members that partici- brane. pate in plasmid 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- eukaryotes, 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 gene 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 Cell Biology 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 Bacillus 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 plasmids 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 treadmilling-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