Cytoskeletal structures are dynamically remodeled with the aid of regulatory proteins. FtsZ (filamentation temperature-sensitive Z) is the bacterial homolog of tubulin that polymerizes into rings localized to cell-division sites, and the constriction of these rings drives cytokinesis. Here we investigate the mechanism by which the Bacillus subtilis cell-division inhibitor, MciZ (mother cell inhibitor of FtsZ), blocks assembly of FtsZ. The X-ray crystal structure reveals that MciZ binds to the C-terminal polymerization interface of FtsZ, the equivalent of the minus end of tubulin. Using in vivo and in vitro assays and microscopy, we show that MciZ, at substoichiometric levels to FtsZ, causes shortening of protofilaments and blocks the assembly of higher-order FtsZ structures. The findings demonstrate an unanticipated capping-based regulatory mechanism for FtsZ.T he discovery that bacteria have actin-, tubulin-, and intermediate filament-like proteins demonstrated that the cytoskeleton is an ancient invention, predating the divergence between prokaryotes and eukaryotes (1). The GTPase FtsZ (filamentation temperature-sensitive Z) was the first prokaryotic protein to be recognized as a cytoskeletal element (2, 3). FtsZ is a tubulin-like protein, which is widely conserved in bacteria and the main component of the bacterial cytokinesis machine, or "divisome." FtsZ self-assembles into single-stranded protofilaments and these associate further inside cells to form a superstructure known as the Z ring (4, 5). FtsZ alone can generate a constriction force to initiate division (6). The Z ring also provides a scaffold onto which several other components of the divisome-mostly cell wall synthesizing enzymes-are recruited and oriented so as to build the division septum, a cross-wall separating a progenitor cell into two isogenic daughter cells (7).FtsZ and tubulin share several essential properties: their assembly is cooperative, stimulated by GTP, and leads to GTP hydrolysis; they form dynamic polymers whose turnover is dependent on nucleotide hydrolysis (8); they use essentially the same bond for polymer formation (9); and recent evidence indicates that they undergo similar allosteric transitions upon polymerization (10, 11). Not surprisingly, however, the functional specialization of these proteins led to some significant differences between them, the most prominent being that FtsZ exists as single protofilaments, whereas tubulin always adopts a multifilament tubular structure. This difference in their higherorder structure implies that the reactions that lead to cooperativity and subunit turnover are likely different. It has also represented a significant technical challenge for the study of FtsZ. Because FtsZ filaments are smaller than the resolution of optical microscopy, so far it has been impossible to determine essential properties associated with its dynamic behavior.Similarly to actin filaments and microtubules, the assembly of FtsZ protofilaments into a Z ring is regulated by a number of proteins that bind directly...

No abstract