No abstract

Cytokinesis defines the last stage in the division cycle, in which cell constriction leads to the formation of daughter cells. The biochemical mechanisms responsible for this process are poorly understood. In bacteria, the ftsZ gene product, FtsZ, is required for cell division, playing a prominent role in cytokinesis. The cellular concentration of FtsZ regulates the frequency of division and genetic studies have indicated that it is the target of several endogenous division inhibitors. At the time of onset of septal invagination, the FtsZ protein is recruited from the cytoplasm to the division site, where it assembles into a ring that remains associated with the leading edge of the invaginating septum until septation is completed. Here we report that FtsZ specifically binds and hydrolyses GTP. The reaction can be dissociated into a GTP-dependent activation stage that is markedly affected by the concentration of FtsZ, and a hydrolysis stage in which GTP is hydrolysed to GDP. The results indicate that GTP binding and hydrolysis are important in enabling FtsZ to support bacterial cytokinesis, either by facilitating the assembly of the FtsZ ring and/or by catalysing an essential step in the cytokinetic process itself.

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 is mediated by the MinC, MinD and MinE proteins. During the normal division cycle MinD plays two roles. It activates the MinC‐dependent mechanism that is responsible for the inactivation of potential division sites and it also renders the division inhibition system sensitive to the topological specificity factor MinE. MinE suppresses the division block at the normal division site at mid‐cell but not all cell poles, thereby ensuring the normal division pattern. In this study the MinD protein was purified to homogeneity and shown to bind ATP and to have ATPase activity. When the putative ATP binding domain of MinD was altered by site‐directed mutagenesis, the mutant protein was no longer able to activate the MinC‐dependent division inhibition system. Immunoelectron microscopy showed that MinD was located in the inner membrane region of the cell envelope. These results show that MinD is a membrane ATPase and suggest that the ATPase activity plays an essential role in the functions of the MinD protein during the normal division process.

The proper placement of the cell division site in Escherichia coli requires the site-specific inactivation of potential division sites at the ceil 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.In Escherichia coli, correct placement of the division septum involves selection of the proper site at midcell. This requires the site-specific inhibition of septation at potential septation sites that are present at the cell poles (9, 11). This process is controlled by the products of the minB operon, MinC, MinD, and MinE (12,13). We have previously shown that coexpression of minC and minD in the absence of MinE leads to the inhibition of septation at both polar and cellinternal potential division sites. Under normal conditions, the MinC/MinD division block is counteracted in a topologically specific fashion by MinE, so that septation is allowed at midcell but is still prevented at the cell poles. The absence of functional MinC or MinD protein or overexpression of minE leads to loss of this site-specific division inhibition process. This results in the frequent misplacement of division septa at cell poles, leading to the formation of small chromosomeless minicells (13).Further insight into the roles of the Min proteins came from the finding (14, 21) that MinC also plays an essential role in the division inhibition that results from expression of the dicB gene, a division inhibitor gene that is normally not expressed (1, 2). MinC/DicB division inhibition differs from MinC/MinD-mediated inhibition in that it is resistant to MinE (14). Because MinC is the common component of both the MinD-and DicB-dependent division inhibition systems, it was suggested that MinC is the component that is responsible for the division block in both systems, with MinD and DicB functioning as activators of the MinC-dependent division inhibitor (14).A candidate for the target of the division inhibitor is the ftsZ gene product. The ftsZ gene is an essential cell division gene which is part of a large cluster of genes at 2 min on the E. coli chromosome that are involved in murein metabolism and septum formation (11,16,22 ...

In Escherichia coli, selection of the proper division site at midcell requires the specific inhibition of septation at two other potential division sites, located at each of the cell poles. This site-specific inhibition of septation is mediated by the gene products of the minicell locus (the minB operon) that includes three genes, minC, minD, and minE. In this paper we show that one of the components of this divisioninhibition system, the minC gene product, is also an essential component of another division-inhibition system, which is induced by derepression of the dicB gene and leads to inhibition of septation at all potential division sites. The two minCdependent division-inhibition systems could be functionally distinguished by their different responses to the minE gene product. The results suggest a model in which a common mechanism, mediated by MinC, is responsible for the division block in a class of division-inhibition systems that can be independently activated by different proteins that determine the specific properties of these systems.Cell division in Escherichia coli is a complex process that must be regulated at several levels. Temporal regulation is required to ensure that septum formation not occur before chromosome replication is completed, and topological regulation is required to ensure that the septum is formed at the midpoint of the cell to permit the equipartition of cytosolic components into daughter cells. One way in which this is accomplished is by the controlled production of endogenous cell division inhibitors.It has been shown (1) that such an inhibitor acts during normal cell growth to ensure that septation is limited to the proper site at midcell. This site-specific inhibition system is a product of the minicell genetic locus (the minB operon) that includes three genes, minC, minD, and minE (minCDE). Under normal conditions, coordinate expression of minC and minD leads to formation ofa potent cell division inhibitor that is given topological specificity by MinE. As a result, septation is permitted at midcell but is blocked at two other potential division sites that are located at the cell poles. It has been suggested that the polar sites are remnants of division sites that were present at midcell during preceding cell cycles (1, 2). When MinE is absent or when minC and minD are overexpressed, septation is inhibited at all potential division sites, leading to filamentation. In the absence of minC or minD expression or in the presence of excess MinE, septation is not prevented at the polar sites, resulting in the formation of anucleate minicells. Therefore, the balanced expression of the minCD division inhibitor and the minE gene product are necessary to maintain the normal division pattern (1, 3).Other known proteins that lead to inhibition of cell division in E. coli are only produced -or activated under special circumstances. The best known of the inducible cell division inhibitors is SfiA. This protein is induced as part of the SOS response to DNA damage. As a result, cell ...