MinC Spatially Controls Bacterial Cytokinesis by Antagonizing the Scaffolding Function of FtsZ

FtsZ-ring Architecture and Its Control by MinCD

In bacteria and archaea, the most widespread cell division system is based on the tubulin homologue FtsZ protein, whose filaments form the cytokinetic Z-ring. FtsZ filaments are tethered to the membrane by anchors such as FtsA and SepF and are regulated by accessory proteins. One such set of proteins is responsible for Z-ring's spatiotemporal regulation, essential for the production of two equal-sized daughter cells. Here, we describe how our still partial understanding of the FtsZ-based cell division process has been progressed by visualising near-atomic structures of Z-rings and complexes that control Z-ring positioning in cells, most notably the MinCDE and Noc systems that act by negatively regulating FtsZ filaments. We summarise available data and how they inform mechanistic models for the cell division process.

E. coli Cell Cycle Machinery

Cytokinesis in E. coli is organized by a cytoskeletal element designated the Z ring. The Z ring is formed at midcell by the coalescence of FtsZ filaments tethered to the membrane by interaction of FtsZ's conserved C-terminal peptide (CCTP) with two membrane-associated proteins, FtsA and ZipA. Although interaction between an FtsZ monomer and either of these proteins is of low affinity, high affinity is achieved through avidity - polymerization linked CCTPs interacting with the membrane tethers. The placement of the Z ring at midcell is ensured by antagonists of FtsZ polymerization that are positioned within the cell and target FtsZ filaments through the CCTP. The placement of the ring is reinforced by a protein network that extends from the terminus (Ter) region of the chromosome to the Z ring. Once the Z ring is established, additional proteins are recruited through interaction with FtsA, to form the divisome. The assembled divisome is then activated by FtsN to carry out septal peptidoglycan synthesis, with a dynamic Z ring serving as a guide for septum formation. As the septum forms, the cell wall is split by spatially regulated hydrolases and the outer membrane invaginates in step with the aid of a transenvelope complex to yield progeny cells.

Bacterial Nucleoid Occlusion: Multiple Mechanisms for Preventing Chromosome Bisection During Cell Division

In most bacteria cell division is driven by the prokaryotic tubulin homolog, FtsZ, which forms the cytokinetic Z ring. Cell survival demands both the spatial and temporal accuracy of this process to ensure that equal progeny are produced with intact genomes. While mechanisms preventing septum formation at the cell poles have been known for decades, the means by which the bacterial nucleoid is spared from bisection during cell division, called nucleoid exclusion (NO), have only recently been deduced. The NO theory was originally posited decades ago based on the key observation that the cell division machinery appeared to be inhibited from forming near the bacterial nucleoid. However, what might drive the NO process was unclear. Within the last 10 years specific proteins have been identified as important mediators of NO. Arguably the best studied NO mechanisms are those employed by the Escherichia coli SlmA and Bacillus subtilis Noc proteins. Both proteins bind specific DNA sequences within selected chromosomal regions to act as timing devices. However, Noc and SlmA contain completely different structural folds and utilize distinct NO mechanisms. Recent studies have identified additional processes and factors that participate in preventing nucleoid septation during cell division. These combined data show multiple levels of redundancy as well as a striking diversity of mechanisms have evolved to protect cells against catastrophic bisection of the nucleoid. Here we discuss these recent findings with particular emphasis on what is known about the molecular underpinnings of specific NO machinery and processes.

Beyond force generation: Why is a dynamic ring of FtsZ polymers essential for bacterial cytokinesis?

We propose that the essential function of the most highly conserved protein in bacterial cytokinesis, FtsZ, is not to generate a mechanical force to drive cell division. Rather, we suggest that FtsZ acts as a signal-processing hub to coordinate cell wall synthesis at the division septum with a diverse array of cellular processes, ensuring that the cell divides smoothly at the correct time and place, and with the correct septum morphology. Here, we explore how the polymerization properties of FtsZ, which have been widely attributed to force generation, can also be advantageous in this signal processing role. We suggest mechanisms by which FtsZ senses and integrates both mechanical and biochemical signals, and conclude by proposing experiments to investigate how FtsZ contributes to the remarkable spatial and temporal precision of bacterial cytokinesis.

Protein Patterns and Oscillations on Lipid Monolayers and in Microdroplets

The Min proteins from E.coli position the bacterial cell-division machinery through pole-to-pole oscillations. In vitro, Min protein self-organization can be reconstituted in the presence of a lipid membrane as a catalytic surface. However, Min dynamics have so far not been reconstituted in fully membrane-enclosed volumes. Microdroplets interfaced by lipid monolayers were employed as a simple 3D mimic of cellular compartments to reconstitute Min protein oscillations. We demonstrate that lipid monolayers are sufficient to fulfil the catalytic role of the membrane and thus represent a facile platform to investigate Min protein regulated dynamics of the cell-division protein FtsZ-mts. In particular, we show that droplet containers reveal distinct Min oscillation modes, and reveal a dependence of FtsZ-mts structures on compartment size. Finally, co-reconstitution of Min proteins and FtsZ-mts in droplets yields antagonistic localization, thus demonstrating that droplets indeed support the analysis of complex bacterial self-organization in confined volumes.

Multistability and dynamic transitions of intracellular Min protein patterns

Cells owe their internal organization to self-organized protein patterns, which originate and adapt to growth and external stimuli via a process that is as complex as it is little understood. Here, we study the emergence, stability, and state transitions of multistable Min protein oscillation patterns in live Escherichia coli bacteria during growth up to defined large dimensions. De novo formation of patterns from homogenous starting conditions is observed and studied both experimentally and in simulations. A new theoretical approach is developed for probing pattern stability under perturbations. Quantitative experiments and simulations show that, once established, Min oscillations tolerate a large degree of intracellular heterogeneity, allowing distinctly different patterns to persist in different cells with the same geometry. Min patterns maintain their axes for hours in experiments, despite imperfections, expansion, and changes in cell shape during continuous cell growth. Transitions between multistable Min patterns are found to be rare events induced by strong intracellular perturbations. The instances of multistability studied here are the combined outcome of boundary growth and strongly nonlinear kinetics, which are characteristic of the reaction-diffusion patterns that pervade biology at many scales.

FtsZ Polymers Tethered to the Membrane by ZipA Are Susceptible to Spatial Regulation by Min Waves

Bacterial cell division is driven by an FtsZ ring in which the FtsZ protein localizes at mid-cell and recruits other proteins, forming a divisome. In Escherichia coli, the first molecular assembly of the divisome, the proto-ring, is formed by the association of FtsZ polymers to the cytoplasmic membrane through the membrane-tethering FtsA and ZipA proteins. The MinCDE system plays a major role in the site selection of the division ring because these proteins oscillate from pole to pole in such a way that the concentration of the FtsZ-ring inhibitor, MinC, is minimal at the cell center, thus favoring FtsZ assembly in this region. We show that MinCDE drives the formation of waves of FtsZ polymers associated to bilayers by ZipA, which propagate as antiphase patterns with respect to those of Min as revealed by confocal fluorescence microscopy. The emergence of these FtsZ waves results from the displacement of FtsZ polymers from the vicinity of the membrane by MinCD, which efficiently competes with ZipA for the C-terminal region of FtsZ, a central hub for multiple interactions that are essential for division. The coupling between FtsZ polymers and Min is enhanced at higher surface densities of ZipA or in the presence of crowding agents that favor the accumulation of FtsZ polymers near the membrane. The association of FtsZ polymers to the membrane modifies the response of FtsZ to Min, and comigrating Min-FtsZ waves are observed when FtsZ is free in solution and not attached to the membrane by ZipA. Taken together, our findings show that the dynamic Min patterns modulate the spatial distribution of FtsZ polymers in controlled minimal membranes. We propose that ZipA plays an important role in mid-cell recruitment of FtsZ orchestrated by MinCDE.

Intrinsic characteristics of Min proteins on the cell division of Helicobacter pylori

Helicobacter pylori divides in the human stomach resulting in persistent infections and causing various disorders. Bacterial cell division is precisely coordinated by many molecules, including FtsZ and Min proteins. However, the role of Min proteins in H. pylori division is poorly understood. We investigated the functional characteristics of Min proteins in wild-type HPK5 and five HPK5-derivative mutants using morphological and genetic approaches. All mutants showed a filamentous shape. However, the bacterial cell growth and viability of three single-gene mutants (minC, minD, minE) were similar to that of the wild-type. The coccoid form number was lowest in the minE-disruptant, indicating that MinE contributes to the coccoid form conversion during the stationary phase. Immunofluorescence microscopic observations showed that FtsZ was dispersedly distributed throughout the bacterial cell irrespective of nucleoid position in only minD-disruptants, indicating that MinD is involved in the nucleoid occlusion system. A chase assay demonstrated that MinC loss suppressed FtsZ-degradation, indicating that FtsZ degrades in a MinC-dependent manner. Molecular interactions between FtsZ and Min proteins were confirmed by immunoprecipitation (IP)-western blotting (WB), suggesting the functional cooperation of these molecules during bacterial cell division. This study describes the intrinsic characteristics of Min proteins and provides new insights into H. pylori cell division.

Defining the rate-limiting processes of bacterial cytokinesis

Bacterial cytokinesis is accomplished by the essential 'divisome' machinery. The most widely conserved divisome component, FtsZ, is a tubulin homolog that polymerizes into the 'FtsZ-ring' ('Z-ring'). Previous in vitro studies suggest that Z-ring contraction serves as a major constrictive force generator to limit the progression of cytokinesis. Here, we applied quantitative superresolution imaging to examine whether and how Z-ring contraction limits the rate of septum closure during cytokinesis in Escherichia coli cells. Surprisingly, septum closure rate was robust to substantial changes in all Z-ring properties proposed to be coupled to force generation: FtsZ's GTPase activity, Z-ring density, and the timing of Z-ring assembly and disassembly. Instead, the rate was limited by the activity of an essential cell wall synthesis enzyme and further modulated by a physical divisome-chromosome coupling. These results challenge a Z-ring-centric view of bacterial cytokinesis and identify cell wall synthesis and chromosome segregation as limiting processes of cytokinesis.

MinC/MinD copolymers are not required for Min function

In Escherichia coli, precise placement of the cytokinetic Z ring at midcell requires the concerted action of the three Min proteins. MinD activates MinC, an inhibitor of FtsZ, at least in part, by recruiting it to the membrane and targeting it to the Z ring, while MinE stimulates the MinD ATPase inducing an oscillation that directs MinC/MinD activity away from midcell. Recently, MinC and MinD were shown to form copolymers of alternating dimers of MinC and MinD, and it was suggested that these copolymers are the active form of MinC/MinD. Here, we use MinD mutants defective in binding MinC to generate heterodimers with wild-type MinD that are unable to form MinC/MinD copolymers. Similarly, MinC mutants defective in binding to MinD were used to generate heterodimers with wild-type MinC that are unable to form copolymers. Such heterodimers are active and in the case of MinC were shown to mediate spatial regulation of the Z ring demonstrating that MinC/MinD copolymer formation is not required. Our results are consistent with a model in which a membrane anchored MinC/MinD complex is targeted to the Z ring through the conserved carboxy tail of FtsZ leading to breakage of FtsZ filaments.

An Optimal Free Energy Dissipation Strategy of the MinCDE Oscillator in Regulating Symmetric Bacterial Cell Division

Sustained molecular oscillations are ubiquitous in biology. The obtained oscillatory patterns provide vital functions as timekeepers, pacemakers and spacemarkers. Models based on control theory have been introduced to explain how specific oscillatory behaviors stem from protein interaction feedbacks, whereas the energy dissipation through the oscillating processes and its role in the regulatory function remain unexplored. Here we developed a general framework to assess an oscillator’s regulation performance at different dissipation levels. Using the Escherichia coli MinCDE oscillator as a model system, we showed that a sufficient amount of energy dissipation is needed to switch on the oscillation, which is tightly coupled to the system’s regulatory performance. Once the dissipation level is beyond this threshold, unlike stationary regulators’ monotonic performance-to-cost relation, excess dissipation at certain steps in the oscillating process damages the oscillator’s regulatory performance. We further discovered that the chemical free energy from ATP hydrolysis has to be strategically assigned to the MinE-aided MinD release and the MinD immobilization steps for optimal performance, and a higher energy budget improves the robustness of the oscillator. These results unfold a novel mode by which living systems trade energy for regulatory function.

This paper presents a unique dissipation mode of converting biochemical free energy in ATP to regulatory function through the MinCDE bio-oscillator that marks the mid-cell position for symmetric bacterial cell division. Through assessing the oscillator’s performance-to-cost relation, we demonstrate that some dissipation threshold needs to be satisfied to switch on the oscillation, but the oscillator’s performance can be damaged by excess free energy dissipation, which is distinct from the known monotonic tradeoff relation of stationary regulators. An optimal dissipation strategy has been unveiled: the ATP free energy must be precisely allocated to specific reaction steps for accurate mid-cell recognition, which also coincides with the dynamic requirements for robust oscillation to occur. These discoveries identify an optimizable operation scheme of free energy consumption in biological systems and provide deep insights into the evolution of dynamic regulatory networks.

Evidence That Bacteriophage λ Kil Peptide Inhibits Bacterial Cell Division by Disrupting FtsZ Protofilaments and Sequestering Protein Subunits

The effects of Kil peptide from bacteriophage λ on the assembly of Escherichia coli FtsZ into one subunit thick protofilaments were studied using combined biophysical and biochemical methods. Kil peptide has recently been identified as the factor from bacteriophage λ responsible for the inhibition of bacterial cell division during lytic cycle, targeting FtsZ polymerization. Here, we show that this antagonist blocks FtsZ assembly into GTP-dependent protofilaments, producing a wide distribution of smaller oligomers compared with the average size of the intact protofilaments. The shortening of FtsZ protofilaments by Kil is detectable at concentrations of the peptide in the low micromolar range, the mid-point of the inhibition being close to its apparent affinity for GDP-bound FtsZ. This antagonist not only interferes with FtsZ assembly but also reverses the polymerization reaction. The negative regulation by Kil significantly reduces the GTPase activity of FtsZ protofilaments, and FtsZ polymers assembled in guanosine-5'-[(α,β)-methyleno]triphosphate are considerably less sensitive to Kil. Our results suggest that, at high concentrations, Kil may use an inhibition mechanism involving the sequestration of FtsZ subunits, similar to that described for other inhibitors like the SOS response protein SulA or the moonlighting enzyme OpgH. This mechanism is different from those employed by the division site selection antagonists MinC and SlmA. This work provides new insight into the inhibition of FtsZ assembly by phages, considered potential tools against bacterial infection.

The Min system and other nucleoid-independent regulators of Z ring positioning

Rod-shaped bacteria such as E. coli have mechanisms to position their cell division plane at the precise center of the cell, to ensure that the daughter cells are equal in size. The two main mechanisms are the Min system and nucleoid occlusion (NO), both of which work by inhibiting assembly of FtsZ, the tubulin-like scaffold that forms the cytokinetic Z ring. Whereas NO prevents Z rings from constricting over unsegregated nucleoids, the Min system is nucleoid-independent and even functions in cells lacking nucleoids and thus NO. The Min proteins of E. coli and B. subtilis form bipolar gradients that inhibit Z ring formation most at the cell poles and least at the nascent division plane. This article will outline the molecular mechanisms behind Min function in E. coli and B. subtilis, and discuss distinct Z ring positioning systems in other bacterial species.

Tetracycline hypersensitivity of an ezrA mutant links GalE and TseB (YpmB) to cell division

Cell division in bacteria is initiated by the polymerization of FtsZ into a ring-like structure at midcell that functions as a scaffold for the other cell division proteins. In Bacillus subtilis, the conserved cell division protein EzrA is involved in modulation of Z-ring formation and coordination of septal peptidoglycan synthesis. Here, we show that an ezrA mutant is hypersensitive to tetracycline, even when the tetracycline efflux pump TetA is present. This effect is not related to the protein translation inhibiting activity of tetracycline. Overexpression of FtsL suppresses this phenotype, which appears to be related to the intrinsic low FtsL levels in an ezrA mutant background. A transposon screen indicated that the tetracycline effect can also be suppressed by overproduction of the cell division protein ZapA. In addition, tetracycline sensitivity could be suppressed by transposon insertions in galE and the unknown gene ypmB, which was renamed tseB (tetracycline sensitivity suppressor of ezrA). GalE is an epimerase using UDP-glucose and UDP-N-acetylglucosamine as substrate. Deletion of this protein bypasses the synthetic lethality of zapA ezrA and sepF ezrA double mutations, indicating that GalE influences cell division. The transmembrane protein TseB contains an extracytoplasmic peptidase domain, and a GFP fusion shows that the protein is enriched at cell division sites. A tseB deletion causes a shorter cell phenotype, indicating that TseB plays a role in cell division. Why a deletion of ezrA renders B. subtilis cells hypersensitive for tetracycline remains unclear. We speculate that this phenomenon is related to the tendency of tetracycline analogs to accumulate into the lipid bilayer, which may destabilize certain membrane proteins.

The Nucleoid Occlusion SlmA Protein Accelerates the Disassembly of the FtsZ Protein Polymers without Affecting Their GTPase Activity

Division site selection is achieved in bacteria by different mechanisms, one of them being nucleoid occlusion, which prevents Z-ring assembly nearby the chromosome. Nucleoid occlusion in E. coli is mediated by SlmA, a sequence specific DNA binding protein that antagonizes FtsZ assembly. Here we show that, when bound to its specific target DNA sequences (SBS), SlmA reduces the lifetime of the FtsZ protofilaments in solution and of the FtsZ bundles when located inside permeable giant vesicles. This effect appears to be essentially uncoupled from the GTPase activity of the FtsZ protofilaments, which is insensitive to the presence of SlmA·SBS. The interaction of SlmA·SBS with either FtsZ protofilaments containing GTP or FtsZ oligomers containing GDP results in the disassembly of FtsZ polymers. We propose that SlmA·SBS complexes control the polymerization state of FtsZ by accelerating the disassembly of the FtsZ polymers leading to their fragmentation into shorter species that are still able to hydrolyze GTP at the same rate. SlmA defines therefore a new class of inhibitors of the FtsZ ring different from the SOS response regulator SulA and from the moonlighting enzyme OpgH, inhibitors of the GTPase activity. SlmA also shows differences compared with MinC, the inhibitor of the division site selection Min system, which shortens FtsZ protofilaments by interacting with the GDP form of FtsZ.

Spatial coordination between chromosomes and cell division proteins in Escherichia coli

To successfully propagate, cells need to coordinate chromosomal replication and segregation with cell division to prevent formation of DNA-less cells and cells with damaged DNA. Here, we review molecular systems in Escherichia coli that are known to be involved in positioning the divisome and chromosome relative to each other. Interestingly, this well-studied micro-organism has several partially redundant mechanisms to achieve this task; none of which are essential. Some of these systems determine the localization of the divisome relative to chromosomes such as SlmA-dependent nucleoid occlusion, some localize the chromosome relative to the divisome such as DNA translocation by FtsK, and some are likely to act on both systems such as the Min system and newly described Ter linkage. Moreover, there is evidence that E. coli harbors other divisome-chromosome coordination systems in addition to those known. The review also discusses the minimal requirements of coordination between chromosomes and cell division proteins needed for cell viability. Arguments are presented that cells can propagate without any dedicated coordination between their chromosomes and cell division machinery at the expense of lowered fitness.

FtsZ1/FtsZ2 Turnover in Chloroplasts and the Role of ARC3

Chloroplast division requires filamentation temperature-sensitive Z (FtsZ), a tubulin-like GTPase of cyanobacterial endosymbiotic origin. Plants and algae possess two distinct FtsZ protein families, FtsZ1 and FtsZ2 that co-assemble into a ring (Z-ring) at the division site. Z-ring assembly and disassembly and division site positioning is controlled by both positive and negative factors via their specific interactions with FtsZ1 and FtsZ2. Here we present the in planta analysis of Arabidopsis FtsZ1 and FtsZ2 turnover in the context of a native chloroplast division machinery. Fluorescence recovery after photobleaching analysis was conducted using fluorescently tagged FtsZ at wild-type (WT)-like levels. Rapid photobleaching, low signal-to-noise ratio, and phototropic movements of chloroplasts were overcome by (i) using progressive intervals in time-lapse imaging, (ii) analyzing epidermal rather than stromal chloroplasts, and (iii) employing image stack alignment during postprocessing. In plants of WT background, fluorescence recovery half-times averaged 117 and 325 s for FtsZ1 and FtsZ2, respectively. In plants lacking ARC3, the key negative regulator of FtsZ assembly, the turnover was threefold slower. The findings are discussed in the context of previous results conducted in a heterologous system.

Cytoskeletal pinning controls phase separation in multicomponent lipid membranes

We study the effect of a minimal cytoskeletal network formed on the surface of giant unilamellar vesicles by the prokaryotic tubulin homolog, FtsZ, on phase separation in freestanding lipid membranes. FtsZ has been modified to interact with the membrane through a membrane targeting sequence from the prokaryotic protein MinD. FtsZ with the attached membrane targeting sequence efficiently forms a highly interconnected network on membranes with a concentration-dependent mesh size, much similar to the eukaryotic cytoskeletal network underlying the plasma membrane. Using giant unilamellar vesicles formed from a quaternary lipid mixture, we demonstrate that the artificial membrane-associated cytoskeleton, on the one hand, suppresses large-scale phase separation below the phase transition temperature, and, on the other hand, preserves phase separation above the transition temperature. Our experimental observations support the ideas put forward in our previous simulation study: In particular, the picket fence effect on phase separation may explain why micrometer-scale membrane domains are observed in isolated, cytoskeleton-free giant plasma membrane vesicles, but not in intact cell membranes. The experimentally observed suppression of large-scale phase separation much below the transition temperatures also serves as an argument in favor of the cryoprotective role of the cytoskeleton.

MinD-like ATPase FlhG effects location and number of bacterial flagella during C-ring assembly

The number and location of flagella, bacterial organelles of locomotion, are species specific and appear in regular patterns that represent one of the earliest taxonomic criteria in microbiology. However, the mechanisms that reproducibly establish these patterns during each round of cell division are poorly understood. FlhG (previously YlxH) is a major determinant for a variety of flagellation patterns. Here, we show that FlhG is a structural homolog of the ATPase MinD, which serves in cell-division site determination. Like MinD, FlhG forms homodimers that are dependent on ATP and lipids. It interacts with a complex of the flagellar C-ring proteins FliM and FliY (also FliN) in the Gram-positive, peritrichous-flagellated Bacillus subtilis and the Gram-negative, polar-flagellated Shewanella putrefaciens. FlhG interacts with FliM/FliY in a nucleotide-independent manner and activates FliM/FliY to assemble with the C-ring protein FliG in vitro. FlhG-driven assembly of the FliM/FliY/FliG complex is strongly enhanced by ATP and lipids. The protein shows a highly dynamic subcellular distribution between cytoplasm and flagellar basal bodies, suggesting that FlhG effects flagellar location and number during assembly of the C-ring. We describe the molecular evolution of a MinD-like ATPase into a flagellation pattern effector and suggest that the underappreciated structural diversity of the C-ring proteins might contribute to the formation of different flagellation patterns.

Effects of various kinetic rates of FtsZ filaments on bacterial cytokinesis

Cell morphodynamics during bacterial cytokinesis is extensively investigated by a combination of phase field model for rod-shaped cells and a kinetic description for FtsZ ring maintenance.

LocZ is a new cell division protein involved in proper septum placement in Streptococcus pneumoniae

How bacteria control proper septum placement at midcell, to guarantee the generation of identical daughter cells, is still largely unknown. Although different systems involved in the selection of the division site have been described in selected species, these do not appear to be widely conserved. Here, we report that LocZ (Spr0334), a newly identified cell division protein, is involved in proper septum placement in Streptococcus pneumoniae. We show that locZ is not essential but that its deletion results in cell division defects and shape deformation, causing cells to divide asymmetrically and generate unequally sized, occasionally anucleated, daughter cells. LocZ has a unique localization profile. It arrives early at midcell, before FtsZ and FtsA, and leaves the septum early, apparently moving along with the equatorial rings that mark the future division sites. Consistently, cells lacking LocZ also show misplacement of the Z-ring, suggesting that it could act as a positive regulator to determine septum placement. LocZ was identified as a substrate of the Ser/Thr protein kinase StkP, which regulates cell division in S. pneumoniae. Interestingly, homologues of LocZ are found only in streptococci, lactococci, and enterococci, indicating that this close phylogenetically related group of bacteria evolved a specific solution to spatially regulate cell division.

IMPORTANCE

Bacterial cell division is a highly ordered process regulated in time and space. Recently, we reported that the Ser/Thr protein kinase StkP regulates cell division in Streptococcus pneumoniae, through phosphorylation of several key proteins. Here, we characterized one of the StkP substrates, Spr0334, which we named LocZ. We show that LocZ is a new cell division protein important for proper septum placement and likely functions as a marker of the cell division site. Consistently, LocZ supports proper Z-ring positioning at midcell. LocZ is conserved only among streptococci, lactococci, and enterococci, which lack homologues of the Min and nucleoid occlusion effectors, indicating that these bacteria adapted a unique mechanism to find their middle, reflecting their specific shape and symmetry.

MinCD cell division proteins form alternating copolymeric cytomotive filaments

During bacterial cell division, filaments of the tubulin-like protein FtsZ assemble at midcell to form the cytokinetic Z-ring. Its positioning is regulated by the oscillation of MinCDE proteins. MinC is activated by MinD through an unknown mechanism and prevents Z-ring assembly anywhere but midcell. Here, using X-ray crystallography, electron microscopy and in vivo analyses we show that MinD activates MinC by forming a new class of alternating copolymeric filaments that show similarity to eukaryotic septin filaments A non-polymerising mutation in MinD causes aberrant cell division in E. coli. MinCD copolymers bind to membrane, interact with FtsZ, and are disassembled by MinE. Imaging a functional msfGFP-MinC fusion protein in MinE deleted cells reveals filamentous structures. EM imaging of our reconstitution of the MinCD-FtsZ interaction on liposome surfaces reveals a plausible mechanism for regulation of FtsZ ring assembly by MinCD copolymers.

Toward Spatially Regulated Division of Protocells: Insights into the E. coli Min System from in Vitro Studies

For reconstruction of controlled cell division in a minimal cell model, or protocell, a positioning mechanism that spatially regulates division is indispensable. In Escherichia coli, the Min proteins oscillate from pole to pole to determine the division site by inhibition of the primary divisome protein FtsZ anywhere but in the cell middle. Remarkably, when reconstituted under defined conditions in vitro, the Min proteins self-organize into spatiotemporal patterns in the presence of a lipid membrane and ATP. We review recent progress made in studying the Min system in vitro, particularly focusing on the effects of various physicochemical parameters and boundary conditions on pattern formation. Furthermore, we discuss implications and challenges for utilizing the Min system for division site placement in protocells.

The bacterial cell division regulators MinD and MinC form polymers in the presence of nucleotide

The Min system of proteins, consisting of MinC, MinD and MinE, is essential for normal cell division in Escherichia coli. MinC forms a polar gradient to restrict placement of the division septum to midcell. MinC localization occurs through a direct interaction with MinD, a membrane-associating Par-like ATPase. MinE stimulates ATP hydrolysis by MinD, thereby releasing MinD from the membrane. Here, we show that MinD forms polymers with MinC and ATP without the addition of phospholipids. The topological regulator MinE induces disassembly of MinCD polymers. Two MinD mutant proteins, MinD(K11A) and MinD(ΔMTS15), are unable to form polymers with MinC.

Architecture of the ring formed by the tubulin homologue FtsZ in bacterial cell division

Membrane constriction is a prerequisite for cell division. The most common membrane constriction system in prokaryotes is based on the tubulin homologue FtsZ, whose filaments in E. coli are anchored to the membrane by FtsA and enable the formation of the Z-ring and divisome. The precise architecture of the FtsZ ring has remained enigmatic. In this study, we report three-dimensional arrangements of FtsZ and FtsA filaments in C. crescentus and E. coli cells and inside constricting liposomes by means of electron cryomicroscopy and cryotomography. In vivo and in vitro, the Z-ring is composed of a small, single-layered band of filaments parallel to the membrane, creating a continuous ring through lateral filament contacts. Visualisation of the in vitro reconstituted constrictions as well as a complete tracing of the helical paths of the filaments with a molecular model favour a mechanism of FtsZ-based membrane constriction that is likely to be accompanied by filament sliding.

DOI:http://dx.doi.org/10.7554/eLife.04601.001

Cell division is the process by which new cells are made. It is therefore vital for the growth and development, and the regeneration and repair of damaged tissues. When bacterial and animal cells divide, they must constrict their membrane inwards to split a single cell into two. In most bacteria, this constriction is guided by a ring-like structure that contains filaments of a protein called FtsZ. During cell division, this structure forms around the inside edge of the cell and when it contracts, it pulls the membrane inwards and causes the cell to constrict and eventually divide.

In recent years, this arrangement of FtsZ filaments has been intensively investigated, giving rise to various theories about how it is made and how it works: for example, some recent studies suggested that FtsZ does not form a continuous ring. Nevertheless, many details about the cell division process remain unknown.

Szwedziak, Wang et al. have now investigated this protein ring in two species of bacteria by turning to advanced forms of microscopy to closely observe its structure and how it works. This included mapping the ring in three dimensions. Contrary to earlier reports that the FtsZ ring is discontinuous, in both a bacterium called Caulobacter crescentus and another called Escherichia coli, the ring forms a continuous shape made up of overlapping filaments.

Szwedziak, Wang et al. then increased the levels of two of the ring's main components: the FtsZ protein that forms the filaments and a protein that anchors these filaments to the cell membrane. This caused the modified cells to constrict and divide at extra sites, which resulted in the formation of abnormally small cells. These findings suggest that these two ring components by themselves are able to generate both the structures and force required for cell constriction. This is supported by the fact that when they were introduced into artificial cell-like structures, these proteins spontaneously self-organised into rings and triggered constriction where they formed.

Szwedziak, Wang et al. propose that constriction only starts once the FtsZ protein forms a closed ring and that the ring's overlapping filaments slide along each other to further decrease its diameter and constrict the cell. The degree of filament overlap likely also increases with constriction, requiring filaments to be shortened to maintain sliding. This shortening, along with sliding, could provide a mechanism by which to drive the constriction process.

This work will be followed by even more detailed studies in order to understand the process of bacterial cell division at the atomic scale and how the cell's wall is reshaped during the process. In the long run, intricate knowledge of how a bacterial cell divides might enable the design of new classes of antibiotics targeting the molecular machinery involved.

DOI:http://dx.doi.org/10.7554/eLife.04601.002

RNA localization in bacteria

One of the most important discoveries in the field of microbiology in the last two decades is that bacterial cells have intricate subcellular organization. This understanding has emerged mainly from the depiction of spatial and temporal organization of proteins in specific domains within bacterial cells, e.g., midcell, cell poles, membrane and periplasm. Because translation of bacterial RNA molecules was considered to be strictly coupled to their synthesis, they were not thought to specifically localize to regions outside the nucleoid. However, the increasing interest in RNAs, including non-coding RNAs, encouraged researchers to explore the spatial and temporal localization of RNAs in bacteria. The recent technological improvements in the field of fluorescence microscopy allowed subcellular imaging of RNAs even in the tiny bacterial cells. It has been reported by several groups, including ours that transcripts may specifically localize in such cells. Here we review what is known about localization of RNA and of the pathways that determine RNA fate in bacteria, and discuss the possible cues and mechanisms underlying these distribution patterns.

An intrinsically disordered linker plays a critical role in bacterial cell division

In bacteria, animals, fungi, and many single celled eukaryotes, division is initiated by the formation of a ring of cytoskeletal protein at the nascent division site. In bacteria, the tubulin-like GTPase FtsZ serves as the foundation for the cytokinetic ring. A conserved feature of FtsZ is an intrinsically disordered peptide known as the C-terminal linker. Chimeric experiments suggest the linker acts as a flexible boom allowing FtsZ to associate with the membrane through a conserved C-terminal domain and also modulates interactions both between FtsZ subunits and between FtsZ and modulatory proteins in the cytoplasm.

Plasmonic nanosensors for simultaneous quantification of multiple protein-protein binding affinities

Most of current techniques used for the quantification of protein-protein interactions require the analysis of one pair of binding partners at a time. Herein we present a label-free, simple, fast, and cost-effective route to characterize binding affinities between multiple macromolecular partners simultaneously, using optical dark-field spectroscopy and individual protein-functionalized gold nanorods as sensing elements. Our NanoSPR method could easily become a simple and standard tool in biological, biochemical, and medical laboratories.

Regulation of cell polarity in bacteria

Bacteria are polarized cells with many asymmetrically localized proteins that are regulated temporally and spatially. This spatiotemporal dynamics is critical for several fundamental cellular processes including growth, division, cell cycle regulation, chromosome segregation, differentiation, and motility. Therefore, understanding how proteins find their correct location at the right time is crucial for elucidating bacterial cell function. Despite the diversity of proteins displaying spatiotemporal dynamics, general principles for the dynamic regulation of protein localization to the cell poles and the midcell are emerging. These principles include diffusion-capture, self-assembling polymer-forming landmark proteins, nonpolymer forming landmark proteins, matrix-dependent self-organizing ParA/MinD ATPases, and small Ras-like GTPases.

Effect of the Min system on timing of cell division in Escherichia coli

In Escherichia coli the Min protein system plays an important role in positioning the division site. We show that this system also has an effect on timing of cell division. We do this in a quantitative way by measuring the cell division waiting time (defined as time difference between appearance of a division site and the division event) and the Z-ring existence time. Both quantities are found to be different in WT and cells without functional Min system. We develop a series of theoretical models whose predictions are compared with the experimental findings. Continuous improvement leads to a final model that is able to explain all relevant experimental observations. In particular, it shows that the chromosome segregation defect caused by the absence of Min proteins has an important influence on timing of cell division. Our results indicate that the Min system affects the septum formation rate. In the absence of the Min proteins this rate is reduced, leading to the observed strongly randomized cell division events and the longer division waiting times.

SlmA antagonism of FtsZ assembly employs a two-pronged mechanism like MinCD

Assembly of the Z-ring over unsegregated nucleoids is prevented by a process called nucleoid occlusion (NO), which in Escherichia coli is partially mediated by SlmA. SlmA is a Z ring antagonist that is spatially regulated and activated by binding to specific DNA sequences (SlmA binding sites, SBSs) more abundant in the origin proximal region of the chromosome. However, the mechanism by which SBS bound SlmA (activated form) antagonizes Z ring assembly is controversial. Here, we report the isolation and characterization of two FtsZ mutants, FtsZ-K190V and FtsZ-D86N that confer resistance to activated SlmA. In trying to understand the basis of resistance of these mutants, we confirmed that activated SlmA antagonizes FtsZ polymerization and determined these mutants were resistant, even though they still bind SlmA. Investigation of SlmA binding to FtsZ revealed activated SlmA binds to the conserved C-terminal tail of FtsZ and that the ability of activated SlmA to antagonize FtsZ assembly required the presence of the tail. Together, these results lead to a model in which SlmA binding to an SBS is activated to bind the tail of FtsZ resulting in further interaction with FtsZ leading to depolymerization of FtsZ polymers. This model is strikingly similar to the model for the inhibitory mechanism of the spatial inhibitor MinCD.

Bacteria divide in the middle of the cell by spatially regulating the position of the Z ring, a cytoskeletal element required for cytokinesis. In the model organisms, Escherichia coli and Bacillus subtilis, two negative regulatory systems contribute to this spatial regulation. Both systems contain antagonists of FtsZ assembly that are localized in the cell. In this study we isolated FtsZ mutants resistant to SlmA, which is positioned within the cell by binding to sites asymmetrically distributed around the chromosome. We confirm that SlmA is activated by DNA binding to antagonize FtsZ polymerization in vitro and that the newly isolated mutants are resistant. We also show that SlmA binds to the very conserved tail of FtsZ and that this is required to antagonize FtsZ assembly even though the tail is not required for polymerization. Together, these results highlight the importance of the tail of FtsZ and lead to a model in which SlmA binding to the tail of FtsZ results in further interactions that break the filament. This mechanism is shared with the other spatial regulator and raises the possibility that it may be a common mechanism among spatial regulators of Z ring assembly.

Divided we stand: splitting synthetic cells for their proliferation

With the recent dawn of synthetic biology, the old idea of man-made artificial life has gained renewed interest. In the context of a bottom-up approach, this entails the de novo construction of synthetic cells that can autonomously sustain themselves and proliferate. Reproduction of a synthetic cell involves the synthesis of its inner content, replication of its information module, and growth and division of its shell. Theoretical and experimental analysis of natural cells shows that, whereas the core synthesis machinery of the information module is highly conserved, a wide range of solutions have been realized in order to accomplish division. It is therefore to be expected that there are multiple ways to engineer division of synthetic cells. Here we survey the field and review potential routes that can be explored to accomplish the division of bottom-up designed synthetic cells. We cover a range of complexities from simple abiotic mechanisms involving splitting of lipid-membrane-encapsulated vesicles due to physical or chemical principles, to potential division mechanisms of synthetic cells that are based on prokaryotic division machineries.

Location of dual sites in E. coli FtsZ important for degradation by ClpXP; one at the C-terminus and one in the disordered linker

ClpXP is a two-component ATP-dependent protease that unfolds and degrades proteins bearing specific recognition signals. One substrate degraded by Escherichia coli ClpXP is FtsZ, an essential cell division protein. FtsZ forms polymers that assemble into a large ring-like structure, termed the Z-ring, during cell division at the site of constriction. The FtsZ monomer is composed of an N-terminal polymerization domain, an unstructured linker region and a C-terminal conserved region. To better understand substrate selection by ClpXP, we engineered FtsZ mutant proteins containing amino acid substitutions or deletions near the FtsZ C-terminus. We identified two discrete regions of FtsZ important for degradation of both FtsZ monomers and polymers by ClpXP in vitro. One region is located 30 residues away from the C-terminus in the unstructured linker region that connects the polymerization domain to the C-terminal region. The other region is near the FtsZ C-terminus and partially overlaps the recognition sites for several other FtsZ-interacting proteins, including MinC, ZipA and FtsA. Mutation of either region caused the protein to be more stable and mutation of both caused an additive effect, suggesting that both regions are important. We also observed that in vitro MinC inhibits degradation of FtsZ by ClpXP, suggesting that some of the same residues in the C-terminal site that are important for degradation by ClpXP are important for binding MinC.

Asymmetric constriction of dividing Escherichia coli cells induced by expression of a fusion between two min proteins

The Min system, consisting of MinC, MinD, and MinE, plays an important role in localizing the Escherichia coli cell division machinery to midcell by preventing FtsZ ring (Z ring) formation at cell poles. MinC has two domains, MinCn and MinCc, which both bind to FtsZ and act synergistically to inhibit FtsZ polymerization. Binary fission of E. coli usually proceeds symmetrically, with daughter cells at roughly 180° to each other. In contrast, we discovered that overproduction of an artificial MinCc-MinD fusion protein in the absence of other Min proteins induced frequent and dramatic jackknife-like bending of cells at division septa, with cell constriction predominantly on the outside of the bend. Mutations in the fusion known to disrupt MinCc-FtsZ, MinCc-MinD, or MinD-membrane interactions largely suppressed bending division. Imaging of FtsZ-green fluorescent protein (GFP) showed no obvious asymmetric localization of FtsZ during MinCc-MinD overproduction, suggesting that a downstream activity of the Z ring was inhibited asymmetrically. Consistent with this, MinCc-MinD fusions localized predominantly to segments of the Z ring at the inside of developing cell bends, while FtsA (but not ZipA) tended to localize to the outside. As FtsA is required for ring constriction, we propose that this asymmetric localization pattern blocks constriction of the inside of the septal ring while permitting continued constriction of the outside portion.

Torsion and curvature of FtsZ filaments

FtsZ filaments participate in bacterial cell division, but it is still not clear how their dynamic polymerization and shape exert force on the underlying membrane. We present a theoretical description of individual filaments that incorporates information from molecular dynamic simulations. The structure of the crystallized Methanococcus jannaschii FtsZ dimer was used to model a FtsZ pentamer that showed a curvature and a twist. The estimated bending and torsion angles between monomers and their fluctuations were included in the theoretical description. The MD data also permitted positioning the curvature with respect to the protein coordinates and allowed us to explore the effect of the relative orientation of the preferred curvature with respect to the surface plane. We find that maximum tension is attained when filaments are firmly attached and oriented with their curvature perpendicular to the surface and that the twist serves as a valve to release or to tighten the tension exerted by the curved filaments on the membrane. The theoretical model also shows that the presence of torsion can explain the shape distribution of short filaments observed by Atomic Force Microscopy in previously published experiments. New experiments with FtsZ covalently attached to lipid membranes show that the filament on-plane curvature depends on lipid head charge, confirming the predicted monomer orientation effects. This new model underlines the fact that the combination of the three elements, filament curvature, twist and the strength and orientation of its surface attachment, can modulate the force exerted on the membrane during cell division.

MinCDE exploits the dynamic nature of FtsZ filaments for its spatial regulation

In Escherichia coli, a contractile ring (Z-ring) is formed at midcell before cytokinesis. This ring consists primarily of FtsZ, a tubulin-like GTPase, that assembles into protofilaments similar to those in microtubules but different in their suprastructures. The Min proteins MinC, MinD, and MinE are determinants of Z-ring positioning in E. coli. MinD and MinE oscillate from pole to pole, and genetic and biochemical evidence concludes that MinC positions the Z-ring by coupling its assembly to the oscillations by direct inhibitory interaction. The mechanism of inhibition of FtsZ polymerization and, thus, positioning by MinC, however, is not understood completely. Our in vitro reconstitution experiments suggest that the Z-ring consists of dynamic protofilament bundles in which monomers constantly are exchanged throughout, stochastically creating protofilament ends along the length of the filament. From the coreconstitution of FtsZ with MinCDE, we propose that MinC acts on the filaments in two ways: by increasing the detachment rate of FtsZ-GDP within the filaments and by reducing the attachment rate of FtsZ monomers to filaments by occupying binding sites on the FtsZ filament lattice. Furthermore, our data show that the MinCDE system indeed is sufficient to cause spatial regulation of FtsZ, required for Z-ring positioning.

The Kil peptide of bacteriophage λ blocks Escherichia coli cytokinesis via ZipA-dependent inhibition of FtsZ assembly

Assembly of the essential, tubulin-like FtsZ protein into a ring-shaped structure at the nascent division site determines the timing and position of cytokinesis in most bacteria and serves as a scaffold for recruitment of the cell division machinery. Here we report that expression of bacteriophage λ kil, either from a resident phage or from a plasmid, induces filamentation of Escherichia coli cells by rapid inhibition of FtsZ ring formation. Mutant alleles of ftsZ resistant to the Kil protein map to the FtsZ polymer subunit interface, stabilize FtsZ ring assembly, and confer increased resistance to endogenous FtsZ inhibitors, consistent with Kil inhibiting FtsZ assembly. Cells with the normally essential cell division gene zipA deleted (in a modified background) display normal FtsZ rings after kil expression, suggesting that ZipA is required for Kil-mediated inhibition of FtsZ rings in vivo. In support of this model, point mutations in the C-terminal FtsZ-interaction domain of ZipA abrogate Kil activity without discernibly altering FtsZ-ZipA interactions. An affinity-tagged-Kil derivative interacts with both FtsZ and ZipA, and inhibits sedimentation of FtsZ filament bundles in vitro. Together, these data inspire a model in which Kil interacts with FtsZ and ZipA in the cell to prevent FtsZ assembly into a coherent, division-competent ring structure. Phage growth assays show that kil+ phage lyse ∼30% later than kil mutant phage, suggesting that Kil delays lysis, perhaps via its interaction with FtsZ and ZipA.

Division site positioning in bacteria: one size does not fit all

Spatial regulation of cell division in bacteria has been a focus of research for decades. It has been well studied in two model rod-shaped organisms, Escherichia coli and Bacillus subtilis, with the general belief that division site positioning occurs as a result of the combination of two negative regulatory systems, Min and nucleoid occlusion. These systems influence division by preventing the cytokinetic Z ring from forming anywhere other than midcell. However, evidence is accumulating for the existence of additional mechanisms that are involved in controlling Z ring positioning both in these organisms and in several other bacteria. In some cases the decision of where to divide is solved by variations on a common evolutionary theme, and in others completely different proteins and mechanisms are involved. Here we review the different ways bacteria solve the problem of finding the right place to divide. It appears that a one-size-fits-all model does not apply, and that individual species have adapted a division-site positioning mechanism that best suits their lifestyle, environmental niche and mode of growth to ensure equal partitioning of DNA for survival of the next generation.

The bacterial Min system

A mother cell giving rise to offspring usually needs to choose the site of cytokinesis carefully, as this will determine the size and shape of the daughter cells. Rod-shaped bacteria that divide by binary fission, such as Escherichia coli, often mark their cell division sites at their cell midpoint so that daughter cells are roughly equivalent in size and shape. So how does E. coli know where its middle is? Its cell poles are defined by the previous cell division, but, because E. coli grows by incorporating new cell wall and membrane uniformly along its length, the future cell division site at mid-cell is newly made and has no known pre-existing markers. One way to select the new mid-cell site would be to measure the distance from the two opposing cell poles, using a system that could recognize markers at those poles and define the spot furthest from both markers. This would require that both polar markers act negatively on cell division at equivalent intensities. The result would be a concentration gradient, with the lowest concentration of the negative regulator at the cell midpoint, the greatest distance from both cell poles. It turns out that E. coli and some other rod-shaped bacteria select their cell midpoint using such a negatively acting morphogen gradient, set up by the Min system, which is the focus of this Primer. As is true for many fascinating molecular mechanisms, the first inkling came from the behavior of cells in which this system was broken.

Coupling FtsZ filaments and morphodynamics during bacterial cell division

Cell morphodynamics during bacterial cell division is extensively investigated by a combination of a phase field model for rod-shaped cells and a kinetic description for FtsZ ring maintenance.

MinC, MinD, and MinE drive counter-oscillation of early-cell-division proteins prior to Escherichia coli septum formation

Bacterial cell division initiates with the formation of a ring-like structure at the cell center composed of the tubulin homolog FtsZ (the Z-ring), which acts as a scaffold for the assembly of the cell division complex, the divisome. Previous studies have suggested that the divisome is initially composed of FtsZ polymers stabilized by membrane anchors FtsA and ZipA, which then recruit the remaining division proteins. The MinCDE proteins prevent the formation of the Z-ring at poles by oscillating from pole to pole, thereby ensuring that the concentration of the Z-ring inhibitor, MinC, is lowest at the cell center. We show that prior to septum formation, the early-division proteins ZipA, ZapA, and ZapB, along with FtsZ, assemble into complexes that counter-oscillate with respect to MinC, and with the same period. We propose that FtsZ molecules distal from high concentrations of MinC form relatively slowly diffusing filaments that are bound by ZapAB and targeted to the inner membrane by ZipA or FtsA. These complexes may facilitate the early stages of divisome assembly at midcell. As MinC oscillates toward these complexes, FtsZ oligomerization and bundling are inhibited, leading to shorter or monomeric FtsZ complexes, which become less visible by epifluorescence microscopy because of their rapid diffusion. Reconstitution of FtsZ-Min waves on lipid bilayers shows that FtsZ bundles partition away from high concentrations of MinC and that ZapA appears to protect FtsZ from MinC by inhibiting FtsZ turnover.

A big issue in biology for the past 100 years has been that of how a cell finds its middle. In Escherichia coli, over 20 proteins assemble at the cell center at the time of division. We show that the MinCDE proteins, which prevent the formation of septa at the cell pole by inhibiting FtsZ, drive the counter-oscillation of early-cell-division proteins ZapA, ZapB, and ZipA, along with FtsZ. We propose that FtsZ forms filaments at the pole where the MinC concentration is the lowest and acts as a scaffold for binding of ZapA, ZapB, and ZipA: such complexes are disassembled by MinC and reform within the MinC oscillation period before accumulating at the cell center at the time of division. The ability of FtsZ to be targeted to the cell center in the form of oligomers bound by ZipA and ZapAB may facilitate the early stages of divisome assembly.

Crystal structure of the N-terminal domain of MinC dimerized via domain swapping

Proper cell division at the mid-site of gram-negative bacteria reflects critical regulation by the min system (MinC, MinD and MinE) of the cytokinetic Z ring, which is a polymer composed of FtsZ subunits. MinC and MinD act together to inhibit aberrantly positioned Z-ring formation. MinC consists of two domains: an N-terminal domain (MinCNTD), which interacts with FtsZ and inhibits FtsZ polymerization, and a C-terminal domain (MinCCTD), which interacts with MinD and inhibits the bundling of FtsZ filaments. These two domains reportedly function together, and both are essential for normal cell division. The full-length dimeric structure of MinC from Thermotoga maritima has been reported, and shows that MinC dimerization occurs via MinCCTD; MinCNTD is not involved in dimerization. Here the crystal structure of Escherichia coli MinCNTD (EcoMinCNTD) is reported. EcoMinCNTD forms a dimer via domain swapping between the first β strands in each subunit. It is therefore suggested that the dimerization of full-length EcoMinC occurs via both MinCCTD and MinCNTD, and that the dimerized EcoMinCNTD likely plays an important role in inhibiting aberrant Z-ring localization.

Propagation of MinCDE waves on free-standing membranes

As a spatial modulator of cytokinesis in Escherichia coli, the Min system cooperates with the nucleoid occlusion mechanism to target the divisome assembly towards mid-cell. Based on a reaction-diffusion mechanism powered by ATP (adenosine triphosphate) hydrolysis, the Min proteins propagate in waves on the cell membrane, resulting in oscillations between the cell poles, thus preventing the formation of the division ring everywhere but in the cell centre. The dynamic behaviour of Min proteins has been successfully reconstructed in vitro on supported lipid bilayers (SLBs), reproducing many of the features observed in the cell. However, there has been a marked discrepancy between the speed of propagation of Min protein waves in vitro, compared with the cellular system. A very plausible explanation is the different mobility of proteins on model membranes, compared with the inner membrane of bacteria. To quantitatively demonstrate how membrane diffusion influences Min wave propagation, we compared Min waves on SLBs with free-standing giant unilamellar vesicles (GUV) membranes which display higher fluidity. Intriguingly, the propagation velocity and wavelength on GUVs are three times higher than those reported on supported bilayers, but the wave period is conserved. This suggests that the shorter spatial period of the patterns in vivo might indeed be primarily explained by lower diffusion coefficients of proteins on the bacterial inner membrane.

MinC protein shortens FtsZ protofilaments by preferentially interacting with GDP-bound subunits

The interaction of MinC with FtsZ and its effects on FtsZ polymerization were studied under close to physiological conditions by a combination of biophysical methods. The Min system is a widely conserved mechanism in bacteria that ensures the correct placement of the division machinery at midcell. MinC is the component of this system that effectively interacts with FtsZ and inhibits the formation of the Z-ring. Here we report that MinC produces a concentration-dependent reduction in the size of GTP-induced FtsZ protofilaments (FtsZ-GTP) as demonstrated by analytical ultracentrifugation, dynamic light scattering, fluorescence correlation spectroscopy, and electron microscopy. Our experiments show that, despite being shorter, FtsZ protofilaments maintain their narrow distribution in size in the presence of MinC. The protein had the same effect regardless of its addition prior to or after FtsZ polymerization. Fluorescence anisotropy measurements indicated that MinC bound to FtsZ-GDP with a moderate affinity (apparent KD ∼10 μM at 100 mm KCl and pH 7.5) very close to the MinC concentration corresponding to the midpoint of the inhibition of FtsZ assembly. Only marginal binding of MinC to FtsZ-GTP protofilaments was observed by analytical ultracentrifugation and fluorescence correlation spectroscopy. Remarkably, MinC effects on FtsZ-GTP protofilaments and binding affinity to FtsZ-GDP were strongly dependent on ionic strength, being severely reduced at 500 mM KCl compared with 100 mM KCl. Our results support a mechanism in which MinC interacts with FtsZ-GDP, resulting in smaller protofilaments of defined size and having the same effect on both preassembled and growing FtsZ protofilaments.

Phage 29 phi protein p1 promotes replication by associating with the FtsZ ring of the divisome in Bacillus subtilis

During evolution, viruses have optimized the interaction with host factors to increase the efficiency of fundamental processes such as DNA replication. Bacteriophage 29 protein p1 is a membrane-associated protein that forms large protofilament sheets that resemble eukaryotic tubulin and bacterial filamenting temperature-sensitive mutant Z protein (FtsZ) polymers. In the absence of protein p1, phage 29 DNA replication is impaired. Here we show that a functional fusion of protein p1 to YFP localizes at the medial region of Bacillus subtilis cells independently of other phage-encoded proteins. We also show that 29 protein p1 colocalizes with the B. subtilis cell division protein FtsZ and provide evidence that FtsZ and protein p1 are associated. Importantly, the midcell localization of YFP-p1 was disrupted in a strain that does not express FtsZ, and the fluorescent signal was distributed all over the cell. Depletion of penicillin-binding protein 2B (PBP2B) in B. subtilis cells did not affect the subcellular localization of YFP-p1, indicating that its distribution does not depend on septal wall synthesis. Interestingly, when 29 protein p1 was expressed, B. subtilis cells were about 1.5-fold longer than control cells, and the accumulation of 29 DNA was higher in mutant B. subtilis cells with increased length. We discuss the biological role of p1 and FtsZ in the 29 growth cycle.

In the beginning, Escherichia coli assembled the proto-ring: an initial phase of division

Cell division in Escherichia coli begins by assembling three proteins, FtsZ, FtsA, and ZipA, to form a proto-ring at midcell. These proteins nucleate an assembly of at least 35 components, the divisome. The structuring of FtsZ to form a ring and the processes that effect constriction have been explained by alternative but not mutually exclusive mechanisms. We discuss how FtsA and ZipA provide anchoring of the cytoplasmic FtsZ to the membrane and how a temporal sequence of alternative protein interactions may operate in the maturation and stability of the proto-ring. How the force needed for constriction is generated and how the proto-ring proteins relate to peptidoglycan synthesis remain as the main challenges for future research.

Spatial control of the cell division site by the Min system in Escherichia coli

The Min system of Escherichia coli is involved in mediating placement of the cell division site at the midcell; this is accomplished through partitioning of the cell division inhibitor MinC to the cell poles to block aberrant polar division. The partitioning of MinC is achieved through its interaction with MinDE, which alternates its cellular distribution periodically between opposite cell poles throughout the cell cycle. This dynamic oscillation is the result of intricate molecular interactions occurring between the three Min proteins on the membrane in a spatiotemporal manner. In this minireview, we discuss recent developments in understanding the molecular mechanisms of the E. coli Min system from cellular, biochemical and biophysical perspectives. In addition, we propose a model that involves the balancing of different molecular interactions at different stages of the oscillation cycle.