MinJ (YvjD) is a topological determinant of cell division in Bacillus subtilis

In Bacillus subtilis, FtsZ ring formation and cell division is favoured at the midcell because the inhibitor proteins MinC and MinD are indirectly restricted to the cell poles by the protein DivIVA. Here we identify MinJ, a topological determinant of medial FtsZ positioning that acts as an intermediary between DivIVA and MinD. Due to unrestricted MinD activity, cells mutated for minJ exhibited pleiotropic defects in homologous recombination, swarming motility and cell division. MinJ restricted MinD activity by localizing MinD to the cell poles through direct protein-protein interaction. MinJ itself localized to cell poles in a manner that was dependent on DivIVA. MinJ is conserved in other low G+C Gram-positive bacteria and may be an important component of cell division site selection in these organisms.     

MinC mutants deficient in MinD- and DicB-mediated cell division inhibition due to loss of interaction with MinD, DicB, or a septal component

The min locus encodes a negative regulatory system that limits formation of the cytokinetic Z ring to midcell by preventing its formation near the poles. Of the three Min proteins, MinC is the inhibitor and prevents Z-ring formation by interacting directly with FtsZ. MinD activates MinC by recruiting it to the membrane and conferring a higher affinity on the MinCD complex for a septal component. MinE regulates the cellular location of MinCD by inducing MinD, and thereby MinC, to oscillate between the poles of the cell, resulting in a time-averaged concentration of MinCD on the membrane that is lowest at midcell. MinC can also be activated by the prophage-encoded protein DicB, which targets MinC to the septum without recruiting it first to the membrane. Previous studies have shown that the C-terminal domain of MinC is responsible for the interaction with MinD, DicB, and the septal component. In the present study, we isolated mutations in the C-terminal domain of MinC that affected its interaction with MinD, DicB, and the septal component. Among the mutations isolated, R133A and S134A are specifically deficient in the interaction with MinD, E156A is primarily affected in the interaction with DicB, and R172A is primarily deficient in the interaction with the septum. These mutations differentiate the interactions of MinC with its partners and further support the model of MinCD- and MinC-DicB-mediated cell division inhibition.     

Isolation and characterization of topological specificity mutants of minD in Bacillus subtilis

In rod-shaped bacteria such as Bacillus subtilis, division site selection is mediated by MinC and MinD, which together function as a division inhibitor. Topological specificity is imposed by DivIVA, which ensures that MinCD specifically inhibits division close to the cell poles, while allowing division at mid-cell. MinD plays a central role in this process, as it positions and activates MinC and is dependent on DivIVA for its own positioning at the poles. To investigate MinD activities further, we have constructed and analysed a collection of minD mutants. Mutations in the conserved ATPase motifs lead to an inactive protein, possibly unable to oligomerize, but which nevertheless retains some affinity for the cell membrane. Several mutations affecting the mid- to C-terminal parts of MinD led to a protein probably unable to interact with DivIVA, but that could still stimulate division inhibition by MinC. These findings suggest that the ATPase activity of MinD is necessary for all its functions (possibly in part by controlling the oligomerization state of the protein). The other mutations may identify a surface of MinD involved in its interactions with DivIVA and a possible mechanism for control of MinD by DivIVA.     

Early targeting of Min proteins to the cell poles in germinated spores of Bacillus subtilis: evidence for division apparatus-independent recruitment of Min proteins to the division site

The earliest event in bacterial cell division is the assembly of a tubulin-like protein, FtsZ, at mid-cell to form a ring. In rod-shaped bacteria, the Min system plays an important role in division site placement by inhibiting FtsZ ring formation specifically at the polar regions of the cell. The Min system comprises MinD and MinC, which form an inhibitor complex and, in Bacillus subtilis, DivIVA, which ensures that division is inhibited only in the polar regions. All three proteins localize to the division site at mid-cell and to cell poles. Their recruitment to the division site is dependent on localization of both 'early' and 'late' division proteins. We have examined the temporal and spatial localization of DivIVA relative to that of FtsZ during the first and second cell division after germination and outgrowth of B. subtilis spores. We show that, although the FtsZ ring assembles at mid-cell about halfway through the cell cycle, DivIVA assembles at this site immediately before cell division and persists there during Z-ring constriction and completion of division. We also show that both DivIVA and MinD localize to the cell poles immediately upon spore germination, well before a Z ring forms at mid-cell. Furthermore, these proteins were found to be present in mature, dormant spores. These results suggest that targeting of Min proteins to division sites does not depend directly on the assembly of the division apparatus, as suggested previously, and that potential polar division sites are blocked at the earliest possible stage in the cell cycle in germinated spores as a mechanism to ensure that equal-sized daughter cells are produced upon cell division.     

The C-terminal domain of MinC inhibits assembly of the Z ring in Escherichia coli

In Escherichia coli, the Min system, consisting of three proteins, MinC, MinD, and MinE, negatively regulates FtsZ assembly at the cell poles, helping to ensure that the Z ring will assemble only at midcell. Of the three Min proteins, MinC is sufficient to inhibit Z-ring assembly. By binding to MinD, which is mostly localized at the membrane near the cell poles, MinC is sequestered away from the cell midpoint, increasing the probability of Z-ring assembly there. Previously, it has been shown that the two halves of MinC have two distinct functions. The N-terminal half is sufficient for inhibition of FtsZ assembly, whereas the C-terminal half of the protein is required for binding to MinD as well as to a component of the division septum. In this study, we discovered that overproduction of the C-terminal half of MinC (MinC(122-231)) could also inhibit cell division and that this inhibition was at the level of Z-ring disassembly and dependent on MinD. We also found that fusing green fluorescent protein to either the N-terminal end of MinC(122-231), the C terminus of full-length MinC, or the C terminus of MinC(122-231) perturbed MinC function, which may explain why cell division inhibition by MinC(122-231) was not detected previously. These results suggest that the C-terminal half of MinC has an additional function in the regulation of Z-ring assembly.     

The conserved C-terminal tail of FtsZ is required for the septal localization and division inhibitory activity of MinCC/MinD

The Escherichia coli Min system contributes to spatial regulation of cytokinesis by preventing assembly of the Z ring away from midcell. MinC is a cell division inhibitor whose activity is spatially regulated by MinD and MinE. MinC has two functional domains of similar size, both of which have division inhibitory activity in the proper context. However, the molecular mechanism of the inhibitory action of either domain is not very clear. Here, we report that the septal localization and division inhibitory activity of MinC^C/MinD requires the conserved C-terminal tail of FtsZ. This tail also mediates interaction with two essential division proteins, ZipA and FtsA, to link FtsZ polymers to the membrane. Overproduction of MinC^C/MinD displaces FtsA from the Z ring and eventually disrupts the Z ring, probably because it also displaces ZipA. These results support a model for the division inhibitory action of MinC/MinD. MinC/MinD binds to ZipA and FtsA decorated FtsZ polymers located at the membrane through the MinC^C/MinD–FtsZ interaction. This binding displaces FtsA and/or ZipA, and more importantly, positions MinC^N near the FtsZ polymers making it a more effective inhibitor.     

A novel component of the division‐site selection system of Bacillus subtilis and a new mode of action for the division inhibitor MinCD

Cell division in bacteria is governed by a complex cytokinetic machinery in which the key player is a tubulin homologue, FtsZ. Most rod‐shaped bacteria divide precisely at mid‐cell between segregated sister chromosomes. Selection of the correct site for cell division is thought to be determined by two negative regulatory systems: the nucleoid occlusion system, which prevents division in the vicinity of the chromosomes, and the Min system, which prevents inappropriate division at the cell poles. In Bacillus subtilis recruitment of the division inhibitor MinCD to cell poles depends on DivIVA, and these proteins were thought to be sufficient for Min function. We have now identified a novel component of the division‐site selection system, MinJ, which bridges DivIVA and MinD. minJ mutants are impaired in division because MinCD activity is no longer restricted to cell poles. Although MinCD was thought to act specifically on FtsZ assembly, analysis of minJ and divIVA mutants showed that their block in division occurs downstream of FtsZ. The results support a model in which the main function of the Min system lies in allowing only a single round of division per cell cycle, and that MinCD acts at multiple levels to prevent inappropriate division.     

Dynamic localization cycle of the cell division regulator MinE in Escherichia coli

The MinC protein directs placement of the division septum to the middle of Escherichia coli cells by blocking assembly of the division apparatus at other sites. MinD and MinE regulate MinC activity by modulating its cellular location in a unique fashion. MinD recruits MinC to the membrane, and MinE induces MinC/MinD to oscillate rapidly between the membrane of opposite cell halves. Using fixed cells, we previously found that a MinE-green fluorescent protein fusion accumulated in an annular structure at or near the midcell, as well as along the membrane on only one side of the ring. Here we show that in living cells, MinE undergoes a rapid localization cycle that appears coupled to MinD oscillation. The results show that MinE is not a fixed marker for septal ring assembly. Rather, they support a model in which MinE stimulates the removal of MinD from the membrane in a wave-like fashion. These waves run from a midcell position towards the poles in an alternating sequence such that the time-averaged concentration of division inhibitor is lowest at midcell.     

ZipA is required for targeting of DMinC/DicB, but not DMinC/MinD, complexes to septal ring assemblies in Escherichia coli

The MinC division inhibitor is required for accurate placement of the septal ring at the middle of the Escherichia coli cell. The N-terminal domain of MinC ((Z)MinC) interferes with FtsZ assembly, while the C-terminal domain ((D)MinC) mediates both dimerization and complex formation with either MinD or DicB. Binding to either of these activators greatly enhances the division-inhibitory activity of MinC in the cell. The MinD ATPase plays a crucial role in the rapid pole-to-pole oscillation of MinC that is proposed to force FtsZ ring formation to midcell. DicB is encoded by one of the cryptic prophages on the E. coli chromosome (Qin) and is normally not synthesized. Binding of MinD or DicB to (D)MinC produces complexes that have high affinities for one or more septal ring-associated targets. Here we show that the FtsZ-binding protein ZipA is required for both recruitment of the (D)MinC/DicB complex to FtsZ rings and the DicB-inducible division block normally seen in MinC(+) cells. In contrast, none of the known FtsZ-associated factors, including ZipA, FtsA, and ZapA, appear to be specifically required for targeting of the (D)MinC/MinD complex to rings, implying that the two MinC/activator complexes must recognize distinct features of FtsZ assemblies. MinD-dependent targeting of MinC may occur in two steps of increasing topological specificity: (i) recruitment of MinC from the cytoplasm to the membrane, and (ii) specific targeting of the MinC/MinD complex to nascent septal ring assemblies on the membrane. Using membrane-tethered derivatives of MinC, we obtained evidence that both of these steps contribute to the efficiency of MinC/MinD-mediated division inhibition.     

Expression of Escherichia coli Min system in Bacillus subtilis and its effect on cell division

In both rod-shaped Bacillus subtilis and Escherichia coli cells, Min proteins are involved in the regulation of division septa formation. In E. coli , dynamic oscillation of MinCD inhibitory complex and MinE, a topological specificity protein, prevents improper polar septation. However, in B. subtilis no MinE is present and no oscillation of Min proteins can be observed. The function of MinE is substituted by that of an unrelated DivIVA protein, which targets MinCD to division sites and retains them at the cell poles. We inspected cell division when the E. coli Min system was introduced into B. subtilis cells. Expression of these heterologous Min proteins resulted in cell elongation. We demonstrate here that E. coli MinD can partially substitute for the function of its B. subtilis protein counterpart. Moreover, E. coli MinD was observed to have similar helical localization as B. subtilis MinD.     

Analysis of MinC reveals two independent domains involved in interaction with MinD and FtsZ

In Escherichia coli FtsZ assembles into a Z ring at midcell while assembly at polar sites is prevented by the min system. MinC, a component of this system, is an inhibitor of FtsZ assembly that is positioned within the cell by interaction with MinDE. In this study we found that MinC consists of two functional domains connected by a short linker. When fused to MalE the N-terminal domain is able to inhibit cell division and prevent FtsZ assembly in vitro. The C-terminal domain interacts with MinD, and expression in wild-type cells as a MalE fusion disrupts min function, resulting in a minicell phenotype. We also find that MinC is an oligomer, probably a dimer. Although the C-terminal domain is clearly sufficient for oligomerization, the N-terminal domain also promotes oligomerization. These results demonstrate that MinC consists of two independently functioning domains: an N-terminal domain capable of inhibiting FtsZ assembly and a C-terminal domain responsible for localization of MinC through interaction with MinD. The fusion of these two independent domains is required to achieve topological regulation of Z ring assembly.     

Topological regulation of cell division in Escherichia coli involves rapid pole to pole oscillation of the division inhibitor MinC under the control of MinD and MinE

Placement of the Z ring at midcell in Escherichia coli is assured by the action of the min system, which blocks usage of potential division sites that exist at the cell poles. This activity of min is achieved through the action of an inhibitor of division, MinC, that is activated by MinD and topologically regulated by MinE. In this study, we have used a functional GFP-MinC fusion to monitor the location of MinC. We find that GFP-MinC is a cytoplasmic protein in the absence of the other Min proteins. The addition of MinD, a peripheral membrane protein that interacts with MinC, results in GFP-MinC appearing on the membrane. In the presence of both MinD and MinE, GFP-MinC oscillates rapidly between the halves of the cell. Thus, MinC is positioned by the other Min products, but in a dynamic manner so that it is in position to inhibit Z ring assembly away from midcell.     

Identification of a polar targeting determinant for Bacillus subtilis DivIVA

The Bacillus subtilis protein DivIVA controls both the positioning of the vegetative cell division site and the polar attachment of the chromosome during sporulation. In vegetative growth DivIVA attracts the bipartite cell division inhibitor MinCD away from the cell centre and towards the cell pole. This process ensures the inactivation of old polar division sites and leaves the cell centre free for the assembly of a new cell division complex. During sporulation MinCD and DivIVA levels fall, but DivIVA remains at the cell poles and becomes involved in the migration of the chromosomes to the pole. In order to investigate polar targeting of DivIVA, we undertook a mutational analysis of the 164-amino-acid protein. These studies identified one mutant (divIVA(R18C)) that could not localize to the cell pole but which retained the ability to support both vegetative growth and 50% sporulation efficiency. Further analysis revealed that, in the absence of polar targeting, DivIVA(R18C) localized to the nucleoid during vegetative growth in a Spo0J/Soj-dependent manner and required Spo0J/Soj and MinD to orientate the chromosomes correctly during sporulation. We demonstrate that polar targeting of DivIVA(R18C) is not essential during vegetative growth because the mutant can recognize the cell division site and influences the localization of MinD. Similarly we show that DivIVA(R18C) can function during sporulation because it can support the Spo0J/Soj orientation of the chromosome. In addition, we establish that both residues 18 and 19 constitute a DivIVA polar targeting determinant.     

Differences in MinC/MinD sensitivity between polar and internal Z rings in Escherichia coli

In Escherichia coli the Z ring has the potential to assemble anywhere along the cell length but is restricted to midcell by the action of negative regulatory systems, including Min. In the current model for the Min system, the MinC/MinD division inhibitory complex is evenly distributed on the membrane and can disrupt Z rings anywhere in the cell; however, MinE spatially regulates MinC/MinD by restricting it to the cell poles, thus allowing Z ring formation at midcell. This model assumes that Z rings formed at different cellular locations have equal sensitivity to MinC/MinD in the absence of MinE. However, here we report evidence that differences in MinC/MinD sensitivity between polar and nonpolar Z rings exists even when there is no MinE. MinC/MinD at proper levels is able to block minicell production in Δmin strains without increasing the cell length, indicating that polar Z rings are preferentially blocked. In the FtsZ-I374V strain (which is resistant to MinC(C)/MinD), wild-type morphology can be easily achieved with MinC/MinD in the absence of MinE. We also show that MinC/MinD at proper levels can rescue the lethal phenotype of a min slmA double deletion mutant, which we think is due to the elimination of polar Z rings (or FtsZ structures), which frees up FtsZ molecules for assembly of Z rings at internal sites to rescue division and growth. Taken together, these data indicate that polar Z rings are more susceptible to MinC/MinD than internal Z rings, even when MinE is absent.     

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.     

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.     

Structural basis for the topological specificity function of MinE

Correct positioning of the division septum in Escherichia coli depends on the coordinated action of the MinC, MinD and MinE proteins. Topological specificity is conferred on the MinCD division inhibitor by MinE, which counters MinCD activity only in the vicinity of the preferred midcell division site. Here we report the structure of the homodimeric topological specificity domain of Escherichia coli MinE and show that it forms a novel alphabeta sandwich. Structure-directed mutagenesis of conserved surface residues has enabled us to identify a spatially restricted site on the surface of the protein that is critical for the topological specificity function of MinE.     

Changes in the Min Oscillation Pattern before and after Cell Birth

The Min system regulates the positioning of the cell division site in many bacteria. In Escherichia coli, MinD migrates rapidly from one cell pole to the other. In conjunction with MinC, MinD helps to prevent unwanted FtsZ rings from assembling at the poles and to stabilize their positioning at midcell. Using time-lapse microscopy of growing and dividing cells expressing a gfp-minD fusion, we show that green fluorescent protein (GFP)-MinD often paused at midcell in addition to at the poles, and the frequency of midcell pausing increased as cells grew longer and cell division approached. At later stages of septum formation, GFP-MinD often paused specifically on only one side of the septum, followed by migration to the other side of the septum or to a cell pole. About the time of septum closure, this irregular pattern often switched to a transient double pole-to-pole oscillation in the daughter cells, which ultimately became a stable double oscillation. The splitting of a single MinD zone into two depends on the developing septum and is a potential mechanism to explain how MinD is distributed equitably to both daughter cells. Septal pausing of GFP-MinD did not require MinC, suggesting that MinC-FtsZ interactions do not drive MinD-septal interactions, and instead MinD recognizes a specific geometric, lipid, and/or protein target at the developing septum. Finally, we observed regular end-to-end oscillation over very short distances along the long axes of minicells, supporting the importance of geometry in MinD localization.Rod-shaped bacteria, such as Escherichia coli, divide by binary fission and thus assemble their cell division apparatus (the divisome) at the cell midpoint. Tubulin-like FtsZ is the major cytoskeletal protein of the divisome (17) and assembles into a polymeric ring on the inner surface of the cytoplasmic membrane (the Z ring). Assembly and eventual contraction of the Z ring are crucial for divisome function, and thus it is not surprising that many regulatory factors control FtsZ assembly (25). Notably, two negatively acting spatial regulatory systems, the Min system and nucleoid occlusion, ensure that the Z ring is located properly at the cell midpoint (18). Whereas a major component of the nucleoid occlusion system can be deleted with no major effects on cell division (2), inactivation of the Min system causes cells to divide either at midcell or aberrantly at cell poles (27). The result of polar cell division is the formation of chromosome-free minicells.The Min system consists of three proteins, MinC, MinD, and MinE (7). MinC has two separate domains, each of which binds to FtsZ and promotes disassembly of FtsZ polymers and polymer bundles (6, 29, 30). MinC also binds to MinD, an ATPase with a carboxy-terminal amphipathic helix that binds to the membrane only when the protein is bound to ATP (11, 12). MinD also forms polymers (31). Finally, MinE is a small protein that binds to MinD and stimulates hydrolysis of its bound ATP in the presence of membranes. By doing so, MinE helps to dislodge MinD from the membrane, although MinE itself can bind to the membrane (10). The result is that MinD and MinE form zones that oscillate from one cell pole to the other, with an oscillation period of seconds to minutes, depending on a number of factors, including temperature (9, 23, 24, 34). In typical cells, MinD spends most of its time bound to the membrane at a cell pole, forming a U-shaped zone, and its transit to the opposite pole is rapid compared to its dwell time (23). MinE typically forms a ring at the edge of the MinD zone (22, 24). The direction of the oscillation is determined strongly by cell geometry (5, 35). Other factors, such as membrane phospholipid composition, also influence MinD oscillation; MinD-ATP preferentially binds anionic phospholipids, such as cardiolipin, which is enriched at cell poles (15, 21, 32).Because MinC binds to MinD, MinC oscillates in concert with MinD and therefore is present at the cell poles for longer times than anywhere else in the cell (13, 22). This sets up a gradient of MinC, with the average smallest amount of MinC at midcell at any one time. The current model is that Z rings are most likely to assemble at the trough of the MinC gradient and are discouraged from assembling at cell poles at the peak of the gradient (14). This is supported by the observation that nonring FtsZ itself oscillates from pole to pole, presumably being chased back and forth by the alternating zones of high MinC concentration (33).However, recent work in Bacillus subtilis has shed new light on the possible function of MinC on the Z ring and the divisome. B. subtilis lacks MinE and thus relies on a static MinC gradient. This is set up by the recruitment of MinC and MinD (MinCD) to the Z ring during formation of the division septum (19, 20). This seems paradoxical, as the presence of MinCD at the Z ring is predicted to destabilize it. However, in B. subtilis, Z rings containing MinCD remain functional. Therefore, MinCD seems to have an important role in preventing the immediate reassembly of Z rings at developing cell poles next to a recently used ring (4, 8).This recruitment of MinCD to the Z ring of B. subtilis prompted us to examine in more detail Min oscillations in E. coli cells undergoing septation. We hypothesized that MinCD might bind to the Z ring at later stages of septation, perhaps helping the Z ring to function by stimulation of FtsZ disassembly. Previous results with green fluorescent protein (GFP)-MinC suggested that MinC could transiently localize to the Z ring during septation (13). Consequently, we tested if MinD, the driving force of the oscillation, could also localize to the Z ring and if this localization was dependent on MinC. We also hypothesized that a more central localization of MinCD during the time of septum formation might explain how Min proteins are partitioned equitably to both daughter cells.     

Localisation of DivIVA by targeting to negatively curved membranes

DivIVA is a conserved protein in Gram-positive bacteria and involved in various processes related to cell growth, cell division and spore formation. DivIVA is specifically targeted to cell division sites and cell poles. In Bacillus subtilis, DivIVA helps to localise other proteins, such as the conserved cell division inhibitor proteins, MinC/MinD, and the chromosome segregation protein, RacA. Little is known about the mechanism that localises DivIVA. Here we show that DivIVA binds to liposomes, and that the N terminus harbours the membrane targeting sequence. The purified protein can stimulate binding of RacA to membranes. In mutants with aberrant cell shapes, DivIVA accumulates where the cell membrane is most strongly curved. On the basis of electron microscopic studies and other data, we propose that this is due to molecular bridging of the curvature by DivIVA multimers. This model may explain why DivIVA localises at cell division sites. A Monte-Carlo simulation study showed that molecular bridging can be a general mechanism for binding of proteins to negatively curved membranes.     

Roles of MinC and MinD in the site-specific septation block mediated by the MinCDE system of Escherichia coli.

The proper placement of the cell division site in Escherichia coli requires the site-specific inactivation of potential division sites at the cell poles in a process that requires the coordinate action of the MinC, MinD, and MinE proteins. In the absence of MinE, the coordinate expression of MinC and MinD leads to a general inhibition of cell division. MinE gives topological specificity to the division inhibition process, so that the septation block is restricted to the cell poles. At normal levels of expression, both MinC and MinD are required for the division block. We show here that, when expressed at high levels, MinC acts as a division inhibitor even in the absence of MinD. The division inhibition that results from MinC overexpression in the absence of MinD is insensitive to the MinE topological specificity factor. The results suggest that MinC is the proximate cause of the septation block and that MinD plays two roles in the MinCDE system--it activates the MinC-dependent division inhibition mechanism and is also required for the sensitivity of the division inhibition system to the MinE topological specificity factor.