The tubulin homolog FtsZ forms a polymeric membrane-associated ring structure (Z ring) at midcell that establishes the site of division and provides an essential framework for the localization of a multiprotein molecular machine that promotes division in Escherichia coli . A number of regulatory proteins interact with FtsZ and modulate FtsZ assembly/disassembly processes, ensuring the spatiotemporal integrity of cytokinesis. The Z-associated proteins (ZapA, ZapB, and ZapC) belong to a group of FtsZ-regulatory proteins that exhibit functionally redundant roles in stabilizing FtsZ-ring assembly by binding and bundling polymeric FtsZ at midcell. In this study, we report the identification of ZapD (YacF) as a member of the E. coli midcell division machinery. Genetics and cell biological evidence indicate that ZapD requires FtsZ but not other downstream division proteins for localizing to midcell, where it promotes FtsZ-ring assembly via molecular mechanisms that overlap with ZapA. Biochemical evidence indicates that ZapD directly interacts with FtsZ and promotes bundling of FtsZ protofilaments. Similarly to ZapA, ZapB, and ZapC, ZapD is dispensable for division and therefore belongs to the growing group of FtsZ-associated proteins in E. coli that aid in the overall fitness of the division process.

The first step in bacterial cytokinesis is the assembly of a stable but dynamic cytokinetic ring made up of the essential tubulin homolog FtsZ at the future site of division. Although FtsZ and its role in cytokinesis have been studied extensively, the precise architecture of the in vivo medial FtsZ ring (Z ring) is not well understood. Recent advances in superresolution imaging suggest that the Z ring comprises short, discontinuous, and loosely bundled FtsZ polymers, some of which are tethered to the membrane. A diverse array of regulatory proteins modulate the assembly, stability, and disassembly of the Z ring via direct interactions with FtsZ. Negative regulators of FtsZ play a critical role in ensuring the accurate positioning of FtsZ at the future site of division and in maintaining Z ring dynamics by controlling FtsZ polymer assembly/disassembly processes. Positive regulators of FtsZ are essential for tethering FtsZ polymers to the membrane and promoting the formation of stabilizing lateral interactions, permitting assembly of a mature Z ring. The past decade has seen the identification of several factors that promote FtsZ assembly, presumably through a variety of distinct molecular mechanisms. While a few of these proteins are broadly conserved, many positive regulators of FtsZ assembly are limited to small groups of closely related organisms, suggesting that FtsZ assembly is differentially modulated across bacterial species. In this review, we focus on the roles of positive regulators in Z ring assembly and in maintaining the integrity of the cytokinetic ring during the early stages of division. Bacterial cytokinesis is mediated by a macromolecular protein machine that is accurately positioned in space and time during the cell cycle. The earliest defined event during cytokinesis is the assembly of a polymeric FtsZ structure at the site of future division known as the FtsZ ring, or Z ring (1). The Z ring serves as a scaffold for the recruitment of other division proteins (2-5). FtsZ, a tubulin homolog, forms homopolymeric linear protofilaments upon binding GTP, and these filaments subsequently disassemble upon hydrolysis of the bound nucleotide (6-9). Approximately 30% of cellular FtsZ is present in the ring, and fluorescence recovery after photobleaching (FRAP) studies reveal that there is dynamic exchange between FtsZ molecules in the ring polymers and FtsZ monomers or short oligomers in the cytoplasm (10, 11).In Escherichia coli and Bacillus subtilis, cytokinesis can be separated chronologically into the following three steps: (i) assembly and stabilization of the Z ring at the future division site and, following a lag, (ii) recruitment of downstream division proteins, many of which are essential to form a constriction-competent division complex, and lastly, (iii) constriction of the Z ring coordinated with synthesis and splitting of septal peptidoglycan, leading to invagination of the cell membrane and division into two daughter cells (Fig. 1A) (2, 12, 13). The force required for constriction is likely...

In Escherichia coli, spatiotemporal control of cell division occurs at the level of the assembly/disassembly process of the essential cytoskeletal protein FtsZ. A number of regulators interact with FtsZ and modulate the dynamics of the assembled FtsZ ring at the midcell division site. In this article, we report the identification of an FtsZ stabilizer, ZapC (Z-associated protein C), in a protein localization screen conducted with E. coli. ZapC colocalizes with FtsZ at midcell and interacts directly with FtsZ, as determined by a protein-protein interaction assay in yeast. Cells lacking or overexpressing ZapC are slightly elongated and have aberrant FtsZ ring morphologies indicative of a role for ZapC in FtsZ regulation. We also demonstrate the ability of purified ZapC to promote lateral bundling of FtsZ in a sedimentation reaction visualized by transmission electron microscopy. While ZapC lacks sequence similarity with other nonessential FtsZ regulators, ZapA and ZapB, all three Zap proteins appear to play an important role in FtsZ regulation during rapid growth. Taken together, our results suggest a key role for lateral bundling of the midcell FtsZ polymers in maintaining FtsZ ring stability during division.

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