In: Biology
What is the mechanism that prevents the FtsZ ring from assembling over the bacterial chromosome during binary fission? What proteins prevent FtsZ from polymerizing at the poles of the cell?
The bacterial nucleoid itself can act as a cell cycle ‘checkpoint’ and prevent division by preventing the formation of FtsZ over the bactrerial chromosomes until the replicated sister chromosomes have segregated—a process termed nucleoid occlusion (a process we will discuss later also). SlmA and Noc are important members manifesting this process. Bernhardt et al. (2005) showed that SlmA is a nucleoid-associated division inhibitor capable of mediating nucleoid occlusion. Nucleoid bound SlmA may compete with membrane-associated FtsZ binding proteins (ZBPs), such as FtsA and ZipA, for binding to FtsZ polymers and thus prevent them from developing into an FtsZ ring. Noc also imparts topological restriction of Z localization or accumulation by blocking cell division in the vicinity of the DNA, leading to correct placement of the division site, reducing the risk of chromosome damage during division.
'Min' family of proteins prevent FTsZ from polarizing at the poles of the cell. There are three members in the family- MinC, D and E. MinD localizes to the membrane only at cell poles and contains an inherent ATPase activity and an ATP-binding domain. MinD is active and only able to bind to the membrane when in its ATP-bound conformation. Once anchored, the protein polymerizes, resulting in clusters of MinD which bind and then activate another protein called MinC, which has activity only when bound by MinD. MinE prevents the MinD/C complex from forming in the center but allows it to stay at the poles. Therefore, the concentration of MinC is higher near the poles of the cell and low near mid-cell. The precise mechanism lies at the heart of a oscillatory behaviour of MinC concentration from poles towards the center regulated by other proteins of the family. MinC directly interacts with FtsZ via its two domains, N-terminal and C-terminal in order to inhibit the assembly of FtsZ protofilaments into higher order structures.The amino-terminal domain is the most potent, shortening existing FtsZ protofilaments. The carboxy-terminal domain of MinC has a weaker inhibitory activity against FtsZ and minimizes the lateral interactions between protofilaments that result in bundles.
Another important spatial regulatory system is nucleoid occlusion factor which also negatively regulates Z-ring assembly, independently of Min system. As a result, Z rings do not form at the cell midpoint until after most of the chromosome has been duplicated and partitioned towards the cell poles. SulA is another protein which is only synthesized when there is DNA damage and prevents Z-ring formation by directly binding to it. After a delay during which DNA gets repaired Lon protease cleaves SulA and thus allows Z rings to assemble.