Splitsville: structural and functional insights into the dynamic bacterial Z ring

R E V I E W S

Splitsville: structural and functional insights into the dynamic bacterial Zring

Daniel P.Haeusser1,2 and William Margolin1

Abstract | Bacteria must divide to increase in number and colonize their niche. Binary fission is themost widespread means of bacterial cell division, but even this relatively simple mechanism has many variations on a theme. In most bacteria, the tubulin homologue FtsZ assembles into a ring structure, termed the Zring, at the site of cytokinesis and recruits additional proteins to forma large protein machinethe divisomethat spans the membrane. In this Review, we discuss current insights into the regulation of the assembly of the Zring and how the divisome drives membrane invagination and septal cell wall growth while flexibly responding to various cellularinputs.

Most bacterial cells divide by binary fission, which is the splitting of a bacterial cell into two progeny cells that are approximately equal in size. Fission is coordinated by a protein complex called the divisome1, the assembly of which is directed by the conserved tubulin homo-logue FtsZ. In several model bacteria, the divisome has been proposed to assemble in temporally distinct stages

(FIG.1a). First, FtsZ undergoes GTP-dependent polymerization into filaments that form a defined ring-like structure (known as the Zring) at the site of cytokinesis. Membrane-associated FtsZ-interacting proteins, such as FtsA and ZipA in Escherichia coli, tether the Zring to the cell envelope, which results in an initial complex calledthe proto-ring (FIG.1b, top). During the second stage, theproto-

ring recruits enzymes that are involved in cell wall (septum) synthesis, such as FtsI in E.coli, and proteins that probably coordinate signalling between these enzymes and the proto-ring, such as FtsN and FtsBLQ, thereby forming the mature divisome (FIG.1a,b,

bottom). The divisome then constricts the cytoplasmic membrane, which in concert with the synthesis of new peptidoglycan at the division septum and invagination of the outer membrane (in Gram-negative bacteria) achieves cytokinesis and cellseparation.

Model bacteria that divide by binary fission include the Gram-negative E.coli and Caulobacter crescentus, as well as the Gram-positive Bacillus subtilis. When cytokinesis is inactivated in these bacteria and similar model organisms, bacterial cells grow as cylindrical rods and chromosomes continue to replicate and segregate, which results in filamentous cells with several nucleoids. Newly avail able genetic and cytological tools...