Bacterial cell division: assembly, maintenance and disassembly of the Z ring

Key Points The first recognizable event in bacterial cell division is the assembly of FtsZ into a ring-like structure at mid-cell. This Z ring serves as a scaffold for the assembly of the division machinery and contracts throughout division, guiding the synthesis of the nascent septum. FtsZ is the ancestral homologue of tubulin and assembles cooperatively, in a GTP-dependent manner, into longitudinal protofilaments similar to those of αβ-tubulin. Although the polymerization of FtsZ in vitro has been studied extensively, little is known about the architecture of the polymers that make up the Z ring in vivo . Importantly, the concentration of FtsZ in vivo is greatly in excess of the critical concentration for assembly. Despite this, the intracellular concentration of FtsZ does not vary greatly during the cell cycle. The initiation of cell division is instead regulated both spatially and temporally at the level of Z ring assembly, by an array of accessory proteins that can modulate the polymerization of FtsZ. FtsZ is a cytoplasmic protein, but it must be tethered to the inner face of the cytoplasmic membrane to form the Z ring. This is achieved primarily by the action of FtsA, which is widely conserved and contains an amphipathic membrane-targeting sequence. Where present, ZipA, an integral membrane protein, can also allow Z ring assembly and, along with ZapA, also contributes to the stability of the Z ring. Other, early-assembling regulators, such as ZapB and SepF, potentially play a part in organizing the ultrastructure of the Z ring and, although it is not normally essential, SepF seems to contribute to this process and might also be able to act as a membrane tether. The Z ring is not a static structure. The polymers that constitute the Z ring are in a state of flux, with subunits rapidly exchanging between the polymer and the cytoplasmic pool. This turnover is stimulated by GTP hydrolysis and is regulated by the antagonistic behaviour of many non-essential proteins, including ZapA, extra Z rings A (EzrA) and ClpX. Several regulatory proteins also render the Z ring responsive to variations in the cell cycle by taking advantage of the accessibility of subunits that is afforded by the dynamic nature of the Z ring. UgtP delays cell division by destabilizing the Z ring, and the induction of SulA during the SOS response prevents new Z ring assembly and disassembles existing Z rings. In addition, during sporulation, mother cell inhibitor of FtsZ (MciZ) prevents