Cold Spring Harb Perspect Biol-2010-Shaevitz

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Cold Spring Harb Perspect Biol-2010-Shaevitz

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  • 14, 20102010; doi: 10.1101/cshperspect.a000364 originally published online JulyCold Spring Harb Perspect Biol

    Joshua W. Shaevitz and Zemer Gitai

    The Structure and Function of Bacterial Actin Homologs

    Subject Collection Cell Biology of Bacteria

    Electron CryotomographyElitza I. Tocheva, Zhuo Li and Grant J. Jensen

    Cyanobacterial Heterocysts

    James W. GoldenKrithika Kumar, Rodrigo A. Mella-Herrera and

    Protein Subcellular Localization in BacteriaDavid Z. Rudner and Richard Losick Cell Division in Bacteria

    Synchronization of Chromosome Dynamics and

    Martin Thanbichler

    Their Spatial RegulationPoles Apart: Prokaryotic Polar Organelles and

    Clare L. Kirkpatrick and Patrick H. ViollierMicroscopyAutomated Quantitative Live Cell Fluorescence

    Michael Fero and Kit Pogliano

    MorphogenesisMyxobacteria, Polarity, and Multicellular

    Dale Kaiser, Mark Robinson and Lee KroosHomologsThe Structure and Function of Bacterial Actin

    Joshua W. Shaevitz and Zemer GitaiMembrane-associated DNA Transport Machines

    Briana Burton and David DubnauBiofilms

    Daniel Lpez, Hera Vlamakis and Roberto KolterThe Bacterial Cell Envelope

    WalkerThomas J. Silhavy, Daniel Kahne and Suzanne III Injectisome

    Bacterial Nanomachines: The Flagellum and Type

    Marc Erhardt, Keiichi Namba and Kelly T. HughesCell Biology of Prokaryotic Organelles

    Dorothee Murat, Meghan Byrne and Arash Komeili Live Bacteria CellsSingle-Molecule and Superresolution Imaging in

    Julie S. Biteen and W.E. Moerner

    SegregationBacterial Chromosome Organization and

    Esteban Toro and Lucy Shapiro

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  • The Structure and Function of BacterialActin Homologs

    Joshua W. Shaevitz1 and Zemer Gitai2

    1Department of Physics and the Lewis-Sigler Institute for Integrative Genomics, Princeton University,Princeton, New Jersey 08544

    2Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544

    Correspondence: zgitai@princeton.edu

    During the past decade, the appreciation and understanding of how bacterial cells canbe organized in both space and time have been revolutionized by the identification andcharacterization of multiple bacterial homologs of the eukaryotic actin cytoskeleton.Some of these bacterial actins, such as the plasmid-borne ParM protein, have highly special-ized functions, whereas other bacterial actins, such as the chromosomally encoded MreBprotein, have been implicated in a wide array of cellular activities. In this review we coverour current understanding of the structure, assembly, function, and regulation of bacterialactins. We focus on ParM as a well-understood reductionist model and on MreB as acentral organizer of multiple aspects of bacterial cell biology. We also discuss the outstand-ing puzzles in the field and possible directions where this fast-developing area may progressin the future.

    The discovery of cytoskeletal proteins in bac-teria has fundamentally altered our under-standing of the organization and evolution ofbacteria as cells. Homologs of eukaryotic actinrepresent themostmolecularly and functionallydiverse family of bacterial cytoskeletal elements.Recent phylogenetic studies have identifiedmore than 20 subgroups of bacterial actinhomologs (Derman et al. 2009) (Fig. 1). Manyof these bacterial actins are encoded on extra-chromosomal plasmids, but most bacterial spe-cies with nonspherical morphologies alsoencode chromosomal actin homologs (Danieland Errington 2003). The two earliest proteinsto be characterized as bacterial actins were the

    chromosomal protein MreB (Jones et al. 2001)and the plasmidic protein ParM (Jensen andGerdes 1997). MreB and ParM remain the best-characterized of the bacterial actins and we willthus focus on these two proteins formost of thisarticle.

    The appreciation that bacteria possess actinhomologs only occurred in the past decade.MreB was first identified as a protein involvedin cell shape regulation in Escherichia coli inthe late1980s (Doi et al. 1988). In theearly1990s,pioneering bioinformatic studies identifiedsimilarities in a group of ATPases that have fiveconserved motifs (Bork et al. 1992), a featuredubbed the actin superfamily fold. Although

    Editors: Lucy Shapiro and Richard Losick

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  • this group includes actin and MreB, it also con-tains proteins that do not polymerize into fila-ments, such as sugar kinases like hexokinaseand chaperones like Hsp70. A number of bacte-rial proteins are present in the actin superfamily,including the bacterial cell division protein FtsAwhich interacts with the tubulin homolog FtsZand may or may not form filaments in differentcontexts (van den Ent and Lowe 2000). BecauseMreB did not appear significantly more relatedto actin than these nonfilamentous proteins,the weak sequence similarity with actin waslargely ignored for the better part of a decade.This changed in 2001 when two seminal papersshowed that Bacillus subtilisMreB forms cytos-keletal filaments in vivo (Jones et al. 2001) andthatThermotogamaritimaMreB forms cytoske-letal filaments in vitro (van den Ent et al. 2001).Indeed, structural and biochemical studies ofbothMreB and ParM have convincingly showedthat these proteins closely resemble actin andpolymerize into linear filaments in a nucleotide-dependent manner (Fig. 2).

    Research following the identification ofbacterial cytoskeletal proteins has focused onunderstanding their assembly, regulation, andfunction. Here, we will summarize our currentunderstanding of these issues and highlightthe outstanding questions. We will begin withParM, whose well-characterized assembly anddynamics represent a model for future studiesof all cytoskeletal proteins. We will then focuson MreB, whose diverse activities appear to becentral to the cell biology of many bacterialspecies.

    PARM AS A MODEL FOR STUDYINGBACTERIAL ACTINS

    Perhaps the most completely understood bacte-rial actin is ParM, the plasmid segregating pro-tein from the R1 plasmid. An interdisciplinaryeffort by a number of groups over the last dec-ade has illuminated the details of ParM functionfrom its fundamental molecular properties to areconstitution of plasmid segregation in an

    MreB

    FtsA

    MamK

    AlfA

    Alp7Alp8

    Alp6

    ParM

    Figure 1. The superfamily of bacterial actin homologs. Shown is a phylogenetic tree of the bacterial actinsubfamilies that have been identified to date based on sequence homology. The subfamilies that have beenexperimentally shown to polymerize are labeled and colored. (Courtesy of Joe Pogliano, based on Dermanet al. 2009.)

    J.W. Shaevitz and Z. Gitai

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  • artificial system. This multipronged approachbrings together techniques from biochemistry,molecular biology, imaging, and materials sci-ence to provide an integrated solution to howParM filaments form and how they lead to themovement of DNA. It is likely that this kindof approach will be useful in the study of theother, lesser-understood bacterial actins.

    ParM Function and Mechanism

    Plasmids are naturally occurringmolecular par-asites that often exploit their host cells machi-nery for replication but provide their ownsegregation machinery. By 1997 it was knownthat the low-copy number E. coli plasmid R1is actively partitioned during cell division sothat each daughter cell retains the drug resist-ance conferred by the plasmid (Jensen andGerdes 1997). This partitioning is achieved bythe active positioning of two R1 sister plasmidsto opposite ends of a cell during cell division(Jensen and Gerdes 1997). The machinery thatdirects the two plasmids to opposite cell polesis grouped in a locus termed par. The presenceof this operon in a plasmid lowers the frequencyof plasmid loss during division by several orders

    of magnitude (Gerdes et al. 1985). The Parmachinery consists of three parts: a cis-actingregion of DNA, parC, that acts as a centromere;the ParR protein that binds to 10 repeats withinparC and has kinetichore-like activity; and theactin-homolog ParM, originally labeled as thepartitioning motor, which drives segregationthrough polymerization dynamics (Dam andGerdes 1994).

    The details of plasmid segregation are ele-gant both in their simplicity and their efficiency.Throughout the cytoplasm, ParM filamentsactively lengthen and shorten in a processknown as dynamic instability that somewhatmimics the action of spindle microtubules(Garner et al. 2004). However, when the ParMfilaments are bound to a plasmid via theParR/parC complex they are stabilized againstdepolymerization (Garner et al. 2004). Conse-quently, plasmid bound ParM filaments pro-ceed to elongate as much as they can, which ina rod-shaped cell pushes the plasmids to thecell poles at opposite ends of the cell axis(Garner et al. 2007) (Fig. 3).

    This mechanism was discovered through