A novel membrane anchor for FtsZ is linked to cell wall hydrolysis in Caulobacter crescentus

GTPase activity regulates FtsZ ring positioning in Caulobacter crescentus

Bacterial cell division is crucial for replication and requires careful coordination via proteins collectively called the divisome. The tubulin-like GTPase FtsZ is the master regulator of this process and serves to recruit downstream divisome proteins and regulate their activities. Upon assembling at mid-cell, FtsZ exhibits treadmilling motion driven by GTP binding and hydrolysis. Treadmilling is proposed to play roles in Z-ring condensation and in distribution and regulation of peptidoglycan (PG) cell wall enzymes. FtsZ polymer superstructure and dynamics are central to its function, yet their regulation is incompletely understood. We addressed these gaps in knowledge by evaluating the contribution of GTPase activity to FtsZ's function in vitro and in Caulobacter crescentus cells. We observed that a lethal mutation that abrogates FtsZ GTP hydrolysis impacts FtsZ dynamics and Z-ring positioning, but not constriction. Aberrant Z-ring positioning was due to insensitivity to the FtsZ regulator MipZ when GTPase activity is reduced. Z-ring mislocalization resulted in DNA damage, likely due to constriction over the nucleoid. Collectively, our results indicate that GTP hydrolysis serves primarily to position the Z-ring at mid-cell in Caulobacter. Proper Z-ring localization is required for effective coordination with chromosome segregation to prevent DNA damage and ensure successful cell division.

Synthetic control of actin polymerization and symmetry breaking in active protocells

Nonlinear biomolecular interactions on membranes drive membrane remodeling crucial for biological processes including chemotaxis, cytokinesis, and endocytosis. The complexity of biomolecular interactions, their redundancy, and the importance of spatiotemporal context in membrane organization impede understanding of the physical principles governing membrane mechanics. Developing a minimal in vitro system that mimics molecular signaling and membrane remodeling while maintaining physiological fidelity poses a major challenge. Inspired by chemotaxis, we reconstructed chemically regulated actin polymerization inside vesicles, guiding membrane self-organization. An external, undirected chemical input induced directed actin polymerization and membrane deformation uncorrelated with upstream biochemical cues, suggesting symmetry breaking. A biophysical model incorporating actin dynamics and membrane mechanics proposes that uneven actin distributions cause nonlinear membrane deformations, consistent with experimental findings. This protocellular system illuminates the interplay between actin dynamics and membrane shape during symmetry breaking, offering insights into chemotaxis and other cell biological processes.

A dynamic duo: Understanding the roles of FtsZ and FtsA for Escherichia coli cell division through in vitro approaches

Bacteria divide by binary fission. The protein machine responsible for this process is the divisome, a transient assembly of more than 30 proteins in and on the surface of the cytoplasmic membrane. Together, they constrict the cell envelope and remodel the peptidoglycan layer to eventually split the cell into two. For Escherichia coli, most molecular players involved in this process have probably been identified, but obtaining the quantitative information needed for a mechanistic understanding can often not be achieved from experiments in vivo alone. Since the discovery of the Z-ring more than 30 years ago, in vitro reconstitution experiments have been crucial to shed light on molecular processes normally hidden in the complex environment of the living cell. In this review, we summarize how rebuilding the divisome from purified components - or at least parts of it - have been instrumental to obtain the detailed mechanistic understanding of the bacterial cell division machinery that we have today.

The divergent early divisome: is there a functional core?

The bacterial divisome is a complex nanomachine that drives cell division and separation. The essentiality of these processes leads to the assumption that proteins with core roles will be strictly conserved across all bacterial genomes. However, recent studies in diverse proteobacteria have revealed considerable variation in the early divisome compared with Escherichia coli. While some proteins are highly conserved, their specific functions and interacting partners vary. Meanwhile, different subphyla use clade-specific proteins with analogous functions. Thus, instead of focusing on gene conservation, we must also explore how key functions are maintained during early division by diverging protein networks. An enhanced awareness of these complex genetic networks will clarify the physical and evolutionary constraints of bacterial division.

Anchors: A way for FtsZ filaments to stay membrane bound

Most bacteria use the tubulin homolog FtsZ to organize their cell division. FtsZ polymers initially assemble into mobile complexes that circle around a ring-like structure at the cell midpoint, followed by the recruitment of other proteins that will constrict the cytoplasmic membrane and synthesize septal peptidoglycan to divide the cell. Despite the need for FtsZ polymers to associate with the membrane, FtsZ lacks intrinsic membrane binding ability. Consequently, FtsZ polymers have evolved to interact with the membrane through adaptor proteins that both bind FtsZ and the membrane. Here, we discuss recent progress in understanding the functions of these FtsZ membrane tethers. Some, such as FtsA and SepF, are widely conserved and assemble into varied oligomeric structures bound to the membrane through an amphipathic helix. Other less-conserved proteins, such as EzrA and ZipA, have transmembrane domains, make extended structures, and seem to bind to FtsZ through two separate interactions. This review emphasizes that most FtsZs use multiple membrane tethers with overlapping functions, which not only attach FtsZ polymers to the membrane but also organize them in specific higher-order structures that can optimize cell division activity. We discuss gaps in our knowledge of these concepts and how future studies can address them.

Synthetic control of actin polymerization and symmetry breaking in active protocells

Non-linear biomolecular interactions on the membranes drive membrane remodeling that underlies fundamental biological processes including chemotaxis, cytokinesis, and endocytosis. The multitude of biomolecules, the redundancy in their interactions, and the importance of spatiotemporal context in membrane organization hampers understanding the physical principles governing membrane mechanics. A minimal, in vitro system that models the functional interactions between molecular signaling and membrane remodeling, while remaining faithful to cellular physiology and geometry is powerful yet remains unachieved. Here, inspired by the biophysical processes underpinning chemotaxis, we reconstituted externally-controlled actin polymerization inside giant unilamellar vesicles, guiding self-organization on the membrane. We show that applying undirected external chemical inputs to this system results in directed actin polymerization and membrane deformation that are uncorrelated with upstream biochemical cues, indicating symmetry breaking. A biophysical model of the dynamics and mechanics of both actin polymerization and membrane shape suggests that inhomogeneous distributions of actin generate membrane shape deformations in a non-linear fashion, a prediction consistent with experimental measurements and subsequent local perturbations. The active protocellular system demonstrates the interplay between actin dynamics and membrane shape in a symmetry breaking context that is relevant to chemotaxis and a suite of other biological processes.

Proteomic survey of the DNA damage response in Caulobacter crescentus

The bacterial DNA damage response is a critical, coordinated response to endogenous and exogenous sources of DNA damage. Response dynamics are dependent on coordinated synthesis and loss of relevant proteins. While much is known about its global transcriptional control, changes in protein abundance that occur upon DNA damage are less well characterized at the system level. Here, we perform a proteome-wide survey of the DNA damage response in Caulobacter crescentus . We find that while most protein abundance changes upon DNA damage are readily explained by changes in transcription, there are exceptions. The survey also allowed us to identify the novel DNA damage response factor, YaaA, which has been overlooked by previously published, transcription- focused studies. A similar survey in a Δ lon strain was performed to explore lon's role in DNA damage survival. The Δ lon strain had a smaller dynamic range of protein abundance changes in general upon DNA damage compared to the wild type strain. This system-wide change to the dynamics of the response may explain this strain's sensitivity to DNA damage. Our proteome survey of the DNA damage response provides additional insight into the complex regulation of stress response and nominates a novel response factor that was overlooked in prior studies.

IMPORTANCE

The DNA damage response helps bacteria to react to and potentially survive DNA damage. The mutagenesis induced during this stress response contributes to the development of antibiotic resistance. Understanding how bacteria coordinate their response to DNA damage could help us to combat this growing threat to human health. While the transcriptional regulation of the bacterial DNA damage response has been characterized, this study is the first to our knowledge to assess the proteomic response to DNA damage in Caulobacter .

Peptidoglycan recycling mediated by an ABC transporter in the plant pathogen Agrobacterium tumefaciens

During growth and division, the bacterial cell wall peptidoglycan (PG) is remodelled, resulting in the liberation of PG muropeptides which are typically reinternalized and recycled. Bacteria belonging to the Rhizobiales and Rhodobacterales orders of the Alphaproteobacteria lack the muropeptide transporter AmpG, despite having other key PG recycling enzymes. Here, we show that an alternative transporter, YejBEF-YepA, takes over this role in the Rhizobiales phytopathogen Agrobacterium tumefaciens. Muropeptide import by YejBEF-YepA governs expression of the β-lactamase AmpC in A. tumefaciens, contributing to β-lactam resistance. However, we show that the absence of YejBEF-YepA causes severe cell wall defects that go far beyond lowered AmpC activity. Thus, contrary to previously established Gram-negative models, PG recycling is vital for cell wall integrity in A. tumefaciens. YepA is widespread in the Rhizobiales and Rhodobacterales, suggesting that YejBEF-YepA-mediated PG recycling could represent an important but overlooked aspect of cell wall biology in these bacteria.

Cell division protein FtsZ: from structure and mechanism to antibiotic target

Antimicrobial resistance to virtually all clinically applied antibiotic classes severely limits the available options to treat bacterial infections. Hence, there is an urgent need to develop and evaluate new antibiotics and targets with resistance-breaking properties. Bacterial cell division has emerged as a new antibiotic target pathway to counteract multidrug-resistant pathogens. New approaches in antibiotic discovery and bacterial cell biology helped to identify compounds that either directly interact with the major cell division protein FtsZ, thereby perturbing the function and dynamics of the cell division machinery, or affect the structural integrity of FtsZ by inducing its degradation. The impressive antimicrobial activities and resistance-breaking properties of certain compounds validate the inhibition of bacterial cell division as a promising strategy for antibiotic intervention.

FtsA Regulates Z-Ring Morphology and Cell Wall Metabolism in an FtsZ C-Terminal Linker-Dependent Manner in Caulobacter crescentus

Bacterial cell division requires the assembly of a multiprotein division machinery, or divisome, that remodels the cell envelope to cause constriction. The cytoskeletal protein FtsZ forms a ringlike scaffold for the divisome at the incipient division site. FtsZ has three major regions: a conserved GTPase domain that polymerizes into protofilaments on binding GTP, a C-terminal conserved peptide (CTC) required for binding membrane-anchoring proteins, and a C-terminal linker (CTL) region of varied length and low sequence conservation. Recently, we demonstrated that the CTL regulates FtsZ polymerization properties in vitro and Z-ring structure and cell wall metabolism in vivo In Caulobacter crescentus, an FtsZ variant lacking the CTL (designated ΔCTL) can recruit all known divisome members and drive local cell wall synthesis but has dominant lethal effects on cell wall metabolism. To understand the underlying mechanism of the CTL-dependent regulation of cell wall metabolism, we expressed chimeras of FtsZ domains from C. crescentus and Escherichia coli and observed that the E. coli GTPase domain fused to the C. crescentus CTC phenocopies C. crescentus ΔCTL. By investigating the contributions of FtsZ-binding partners, we identified variants of FtsA, a known membrane anchor for FtsZ, that delay or exacerbate the ΔCTL phenotype. Additionally, we observed that the ΔCTL protein forms extended helical structures in vivo upon FtsA overproduction. We propose that misregulation downstream of defective ΔCTL assembly is propagated through the interaction between the CTC and FtsA. Overall, our study provides mechanistic insights into the CTL-dependent regulation of cell wall enzymes downstream of FtsZ polymerization.IMPORTANCE Bacterial cell division is essential and requires the recruitment and regulation of a complex network of proteins needed to initiate and guide constriction and cytokinesis. FtsZ serves as a master regulator for this process, and its function is highly dependent on both its assembly into the canonical Z ring and interactions with protein binding partners, all of which results in the activation of enzymes that remodel the cell wall to drive constriction. Using mutants of FtsZ, we have elaborated on the role of its C-terminal linker domain in regulating Z-ring stability and dynamics, as well as the requirement for its conserved C-terminal domain and interaction with the membrane-anchoring protein FtsA for regulating the process of cell wall remodeling for constriction.

Non-essentiality of canonical cell division genes in the planctomycete Planctopirus limnophila

Most bacteria divide by binary fission using an FtsZ-based mechanism that relies on a multi-protein complex, the divisome. In the majority of non-spherical bacteria another multi-protein complex, the elongasome, is also required for the maintenance of cell shape. Components of these multi-protein assemblies are conserved and essential in most bacteria. Here, we provide evidence that at least three proteins of these two complexes are not essential in the FtsZ-less ovoid planctomycete bacterium Planctopirus limnophila which divides by budding. We attempted to construct P. limnophila knock-out mutants of the genes coding for the divisome proteins FtsI, FtsK, FtsW and the elongasome protein MreB. Surprisingly, ftsI, ftsW and mreB could be deleted without affecting the growth rate. On the other hand, the conserved ftsK appeared to be essential in this bacterium. In conclusion, the canonical bacterial cell division machinery is not essential in P. limnophila and this bacterium divides via budding using an unknown mechanism.

Bent Bacteria: A Comparison of Cell Shape Mechanisms in Proteobacteria

Helical cell shape appears throughout the bacterial phylogenetic tree. Recent exciting work characterizing cell shape mutants in a number of curved and helical Proteobacteria is beginning to suggest possible mechanisms and provide tools to assess functional significance. We focus here on Caulobacter crescentus, Vibrio cholerae, Helicobacter pylori, and Campylobacter jejuni, organisms from three classes of Proteobacteria that live in diverse environments, from freshwater and saltwater to distinct compartments within the gastrointestinal tract of humans and birds. Comparisons among these bacteria reveal common themes as well as unique solutions to the task of maintaining cell curvature. While motility appears to be influenced in all these bacteria when cell shape is perturbed, consequences on niche colonization are diverse, suggesting the need to consider additional selective pressures.

Agrobacterium tumefaciens divisome proteins regulate the transition from polar growth to cell division

The mechanisms that restrict peptidoglycan biosynthesis to the pole during elongation and re-direct peptidoglycan biosynthesis to mid-cell during cell division in polar-growing Alphaproteobacteria are largely unknown. Here, we explore the role of early division proteins of Agrobacterium tumefaciens including three FtsZ homologs, FtsA and FtsW in the transition from polar growth to mid-cell growth and ultimately cell division. Although two of the three FtsZ homologs localize to mid-cell, exhibit GTPase activity and form co-polymers, only one, FtsZ_AT , is required for cell division. We find that FtsZ_AT is required not only for constriction and cell separation, but also for initiation of peptidoglycan synthesis at mid-cell and cessation of polar peptidoglycan biosynthesis. Depletion of FtsZ_AT in A. tumefaciens causes a striking phenotype: cells are extensively branched and accumulate growth active poles through tip splitting events. When cell division is blocked at a later stage by depletion of FtsA or FtsW, polar growth is terminated and ectopic growth poles emerge from mid-cell. Overall, this work suggests that A. tumefaciens FtsZ makes distinct contributions to the regulation of polar growth and cell division.

ZipN is an essential FtsZ membrane tether and contributes to the septal localization of SepJ in the filamentous cyanobacterium Anabaena

The organismic unit of heterocyst-forming cyanobacteria is a filament of communicating cells connected by septal junctions, proteinaceous structures bridging the cytoplasms of contiguous cells. This distinct bacterial organization is preserved during cell division. In Anabaena, deletion of the zipN gene could not be segregated. We generated strain CSL109 that expresses zipN from a synthetic regulatable promoter. Under conditions of ZipN depletion, cells progressively enlarged, reflecting restricted cell division, and showed drastic morphological alterations including cell detachment from the filaments, to finish lysing. In contrast to the wild-type localization in midcell Z-rings, FtsZ was found in delocalized aggregates in strain CSL109. Consistently, the proportion of membrane-associated to soluble FtsZ in fractionated cell extracts was lower in CSL109. Bacterial two-hybrid analysis showed that ZipN interacts with FtsZ and other cell-division proteins including cytoplasmic Ftn6 and SepF, and polytopic FtsW, FtsX, FtsQ and FtsI. Additionally, ZipN interacted with the septal protein SepJ, and in CSL109 depletion of ZipN was concomitant with a progressive loss of septal specificity of SepJ. Thus, in Anabaena ZipN represents an essential FtsZ membrane tether and an organizer of the divisome, and it contributes to the conformation of septal structures for filament integrity and intercellular communication.

Direct Interaction between the Two Z Ring Membrane Anchors FtsA and ZipA

The initiation of Escherichia coli cell division requires three proteins, FtsZ, FtsA, and ZipA, which assemble in a dynamic ring-like structure at midcell. Along with the transmembrane protein ZipA, the actin-like FtsA helps to tether treadmilling polymers of tubulin-like FtsZ to the membrane. In addition to forming homo-oligomers, FtsA and ZipA interact directly with the C-terminal conserved domain of FtsZ. Gain-of-function mutants of FtsA are deficient in forming oligomers and can bypass the need for ZipA, suggesting that ZipA may normally function to disrupt FtsA oligomers, although no direct interaction between FtsA and ZipA has been reported. Here, we use in vivo cross-linking to show that FtsA and ZipA indeed interact directly. We identify the exposed surface of FtsA helix 7, which also participates in binding to ATP through its internal surface, as a key interface needed for the interaction with ZipA. This interaction suggests that FtsZ's membrane tethers may regulate each other's activities.IMPORTANCE To divide, most bacteria first construct a protein machine at the plane of division and then recruit the machinery that will synthesize the division septum. In Escherichia coli, this first stage involves the assembly of FtsZ polymers at midcell, which directly bind to membrane-associated proteins FtsA and ZipA to form a discontinuous ring structure. Although FtsZ directly binds both FtsA and ZipA, it is unclear why FtsZ requires two separate membrane tethers. Here, we uncover a new direct interaction between the tethers, which involves a helix within FtsA that is adjacent to its ATP binding pocket. Our findings imply that in addition to their known roles as FtsZ membrane anchors, FtsA and ZipA may regulate each other's structure and function.

Cell division control in Caulobacter crescentus

Caulobacter crescentus is a free-living Alphaproteobacterium that thrives in oligotrophic environments. This review focuses on the regulatory network used by this bacterium to control the levels of cell division proteins, their organization inside the cell and their activity as a function of the cell cycle. Strikingly, C. crescentus makes frequent use of master transcriptional regulators and epigenetic signals, most likely to synchronize cell division with other events of the cell cycle. In addition, cellular metabolism and DNA damage sensors emerge as central players regulating cell division in response to changing environmental conditions.

Cell Wall Biogenesis During Elongation and Division in the Plant Pathogen Agrobacterium tumefaciens

A great diversity of bacterial cell shapes can be found in nature, suggesting that cell wall biogenesis is regulated both spatially and temporally. Although Agrobacterium tumefaciens has a rod-shaped morphology, the mechanisms underlying cell growth are strikingly different than other well-studied rod-shaped bacteria including Escherichia coli. Technological advances, such as the ability to deplete essential genes and the development of fluorescent D-amino acids, have enabled recent advances in our understanding of cell wall biogenesis during cell elongation and division of A. tumefaciens. In this review, we address how the field has evolved over the years by providing a historical overview of cell elongation and division in rod-shaped bacteria. Next, we summarize the current understanding of cell growth and cell division processes in A. tumefaciens. Finally, we highlight the need for further research to answer key questions related to the regulation of cell wall biogenesis in A. tumefaciens.

FzlA, an essential regulator of FtsZ filament curvature, controls constriction rate during Caulobacter division

During bacterial division, polymers of the tubulin-like GTPase FtsZ assemble at midcell to form the cytokinetic Z-ring, which coordinates peptidoglycan (PG) remodeling and envelope constriction. Curvature of FtsZ filaments promotes membrane deformation in vitro, but its role in division in vivo remains undefined. Inside cells, FtsZ directs PG insertion at the division plane, though it is unclear how FtsZ structure and dynamics are mechanistically coupled to PG metabolism. Here we study FzlA, a division protein that stabilizes highly curved FtsZ filaments, as a tool for assessing the contribution of FtsZ filament curvature to constriction. We show that in Caulobacter crescentus, FzlA must bind to FtsZ for division to occur and that FzlA-mediated FtsZ curvature is correlated with efficient division. We observed that FzlA influences constriction rate, and that this activity is associated with its ability to bind and curve FtsZ polymers. Further, we found that a slowly constricting fzlA mutant strain develops 'pointy' poles, suggesting that FzlA influences the relative contributions of radial versus longitudinal PG insertion at the septum. These findings implicate FzlA as a critical coordinator of envelope constriction through its interaction with FtsZ and suggest a functional link between FtsZ curvature and efficient constriction in C. crescentus.

Amidase activity is essential for medial localization of AmiC in Caulobacter crescentus

Bacterial cell division is a complex process brought about by the coordinated action of multiple proteins. Separation of daughter cells during the final stages of division involves cleavage of new cell wall laid down at the division septum. In E. coli, this process is governed by the action of N-acetylmuramoyl-L-alanine amidases AmiA/B/C, which are regulated by their LytM activators EnvC and NlpD. While much is known about the regulation of septum cleavage in E. coli, the mechanism of daughter cell separation is not clear in Caulobacter crescentus, a dimorphic crescent-shaped bacterium. In this work, we characterized the role of AmiC, the only annotated amidase in C. crescentus. AmiC from C. crescentus is functional in E. coli and restores cell separation defects seen in E. coli amidase mutants, suggesting that AmiC has septum splitting activity. The medial localization of AmiC was independent of DipM, an LytM domain-containing endopeptidase. Our results indicate that enzymatic activity is essential for medial recruitment of AmiC. Overexpression of AmiC causes cell separation defects and formation of chains. Finally, overexpression of AmiC in cells inhibited for cell division leads to lysis. Collectively, our findings reveal that regulation of daughter cell separation in C. crescentus differs from that of E. coli and can serve as a model system to study bacterial cytokinesis.

The intrinsically disordered C-terminal linker of FtsZ regulates protofilament dynamics and superstructure in vitro

The bacterial tubulin FtsZ polymerizes to form a discontinuous ring that drives bacterial cell division by directing local cell wall synthesis. FtsZ comprises a polymerizing GTPase domain, an intrinsically disordered C-terminal linker (CTL), and a C-terminal conserved peptide (CTC). FtsZ protofilaments align circumferentially in the cell, with the CTC mediating attachment to membrane-associated division proteins. The assembly of FtsZ protofilaments into dynamic clusters is critical for cell division, but the interactions between protofilaments and regulatory mechanisms that mediate cluster assembly and dynamics are unknown. Here, we describe a role for the CTL of Caulobacter crescentus FtsZ as an intrinsic regulator of lateral interactions between protofilaments in vitro FtsZ lacking its CTL (ΔCTL) shows a dramatically increased propensity to form long multifilament bundles compared with wild type (WT). ΔCTL also displays a reduced GTP hydrolysis rate compared with WT, but this altered activity does not account for bundle formation, as reducing protofilament turnover in WT is not sufficient to induce bundling. Surprisingly, binding of the membrane-anchoring protein FzlC disrupts ΔCTL bundling in a CTC-dependent manner. Moreover, the CTL affects the ability of the FtsZ curving protein FzlA to promote formation of helical bundles. We conclude that the CTL of FtsZ influences polymer structure and dynamics both through intrinsic effects on lateral interactions and turnover and by influencing extrinsic regulation of FtsZ by binding partners. Our characterization of CTL function provides a biochemical handle for understanding the relationship between FtsZ-ring structure and function in bacterial cytokinesis.

FtsEX-mediated regulation of the final stages of cell division reveals morphogenetic plasticity in Caulobacter crescentus

During its life cycle, Caulobacter crescentus undergoes a series of coordinated shape changes, including generation of a polar stalk and reshaping of the cell envelope to produce new daughter cells through the process of cytokinesis. The mechanisms by which these morphogenetic processes are coordinated in time and space remain largely unknown. Here we demonstrate that the conserved division complex FtsEX controls both the early and late stages of cytokinesis in C. crescentus, namely initiation of constriction and final cell separation. ΔftsE cells display a striking phenotype: cells are chained, with skinny connections between cell bodies resulting from defects in inner membrane fusion and cell separation. Surprisingly, the thin connections in ΔftsE cells share morphological and molecular features with C. crescentus stalks. Our data uncover unanticipated morphogenetic plasticity in C. crescentus, with loss of FtsE causing a stalk-like program to take over at failed division sites.

Bacterial cell shape is genetically hardwired and is critical for fitness and, in certain cases, pathogenesis. In most bacteria, a semi-rigid structure called the cell wall surrounds the inner membrane, offering protection against cell lysis while simultaneously maintaining cell shape. A highly dynamic macromolecular structure, the cell wall undergoes extensive remodeling as bacterial cells grow and divide. We demonstrate that a broadly conserved cell division complex, FtsEX, relays signals from the cytoplasm to the cell wall to regulate key developmental shape changes in the α-proteobacterium Caulobacter crescentus. Consistent with studies in diverse bacteria, we observe strong synthetic interactions between ftsE and cell wall hydrolytic factors, suggesting that regulation of cell wall remodeling is a conserved function of FtsEX. Loss of FtsE causes morphological defects associated with both the early and late stages of division. Intriguingly, without FtsE, cells frequently fail to separate and instead elaborate a thin, tubular structure between cell bodies, a growth mode observed in other α-proteobacteria. Overall, our results highlight the plasticity of bacterial cell shape and demonstrate how altering the activity of one morphogenetic program can produce diverse morphologies resembling those of other bacteria in nature.

Determinants of Bacterial Morphology: From Fundamentals to Possibilities for Antimicrobial Targeting

Bacterial morphology is extremely diverse. Specific shapes are the consequence of adaptive pressures optimizing bacterial fitness. Shape affects critical biological functions, including nutrient acquisition, motility, dispersion, stress resistance and interactions with other organisms. Although the characteristic shape of a bacterial species remains unchanged for vast numbers of generations, periodical variations occur throughout the cell (division) and life cycles, and these variations can be influenced by environmental conditions. Bacterial morphology is ultimately dictated by the net-like peptidoglycan (PG) sacculus. The species-specific shape of the PG sacculus at any time in the cell cycle is the product of multiple determinants. Some morphological determinants act as a cytoskeleton to guide biosynthetic complexes spatiotemporally, whereas others modify the PG sacculus after biosynthesis. Accumulating evidence supports critical roles of morphogenetic processes in bacteria-host interactions, including pathogenesis. Here, we review the molecular determinants underlying morphology, discuss the evidence linking bacterial morphology to niche adaptation and pathogenesis, and examine the potential of morphological determinants as antimicrobial targets.

Assembly and activation of the Escherichia coli divisome

Cell division in Escherichia coli is mediated by a large protein complex called the divisome. Most of the divisome proteins have been identified, but how they assemble onto the Z ring scaffold to form the divisome and work together to synthesize the septum is not well understood. In this review, we summarize the latest findings on divisome assembly and activation as well as provide our perspective on how these two processes might be regulated.

A New Essential Cell Division Protein in Caulobacter crescentus

Bacterial cell division is a complex process that relies on a multiprotein complex composed of a core of widely conserved and generally essential proteins and on accessory proteins that vary in number and identity in different bacteria. The assembly of this complex and, particularly, the initiation of constriction are regulated processes that have come under intensive study. In this work, we characterize the function of DipI, a protein conserved in Alphaproteobacteria and Betaproteobacteria that is essential in Caulobacter crescentus Our results show that DipI is a periplasmic protein that is recruited late to the division site and that it is required for the initiation of constriction. The recruitment of the conserved cell division proteins is not affected by the absence of DipI, but localization of DipI to the division site occurs only after a mature divisome has formed. Yeast two-hybrid analysis showed that DipI strongly interacts with the FtsQLB complex, which has been recently implicated in regulating constriction initiation. A possible role of DipI in this process is discussed.IMPORTANCE Bacterial cell division is a complex process for which most bacterial cells assemble a multiprotein complex that consists of conserved proteins and of accessory proteins that differ among bacterial groups. In this work, we describe a new cell division protein (DipI) present only in a group of bacteria but essential in Caulobacter crescentus Cells devoid of DipI cannot constrict. Although a mature divisome is required for DipI recruitment, DipI is not needed for recruiting other division proteins. These results, together with the interaction of DipI with a protein complex that has been suggested to regulate cell wall synthesis during division, suggest that DipI may be part of the regulatory mechanism that controls constriction initiation.

Shapeshifting to Survive: Shape Determination and Regulation in Caulobacter crescentus

Bacterial cell shape is a genetically encoded and inherited feature that is optimized for efficient growth, survival, and propagation of bacteria. In addition, bacterial cell morphology is adaptable to changes in environmental conditions. Work in recent years has demonstrated that individual features of cell shape, such as length or curvature, arise through the spatial regulation of cell wall synthesis by cytoskeletal proteins. However, the mechanisms by which these different morphogenetic factors are coordinated and how they may be globally regulated in response to cell cycle and environmental cues are only beginning to emerge. Here, we have summarized recent advances that have been made to understand morphology in the dimorphic Gram-negative bacterium Caulobacter crescentus.

Erin Goley: Catching the bug for studying the cytoskeleton

Erin Goley investigates how the microbial cytoskeleton controls cell growth and division.

Erin Goley investigates how the microbial cytoskeleton controls cell growth and division.

An intracellular compass spatially coordinates cell cycle modules in Caulobacter crescentus

Cellular functions in Bacteria, such as chromosome segregation and cytokinesis, result from cascades of molecular events operating largely as self-contained modules. Regulated timing of these cellular modules stems from global genetic circuits that allow precise temporal activation with respect to cell cycle progression and cell differentiation. Critically, many of these functions occur at defined locations within the cell, and therefore regulators of each module must communicate to remain coordinated in space. In this perspective, we highlight recent discoveries in Caulobacter crescentus asymmetric cell division to illuminate diverse mechanisms by which a cellular compass, composed of scaffolding and signaling proteins, directs cell cycle modules to their exact cellular addresses.