Membrane anchors for the bacterial tubulin FtsZ regulate cell shape during Caulobacter crescentus cell division
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Bacterial cell shape is genetically hardwired and critical for fitness as well as, in certain cases, pathogenesis. In most bacteria, a semi-rigid structure called the cell wall surrounds the inner membrane, offering protection against cell lysis while simultaneously maintaining cell shape. A highly dynamic macromolecular structure, the cell wall undergoes extensive remodeling as bacterial cells grow and divide. In the majority of bacteria, the tubulin-like GTPase FtsZ is essential for division and forms an annulus at midcell (the Z-ring) which recruits the division machinery and regulates cell wall remodeling. Although both activities require membrane attachment of FtsZ, few membrane anchors have been characterized. In the model α-proteobacterium Caulobacter crescentus, the division proteins FzlC and FtsEX arrive at midcell shortly after FtsZ and likely function as early FtsZ membrane anchors. As membrane attachment is important for FtsZ form and function, we sought to characterize the activities of FzlC and FtsEX toward FtsZ during cytokinesis. We demonstrate that FzlC associates with membranes directly in vivo and in vitro and recruits FtsZ to membranes in vitro. In vivo, overproduction of FzlC results in cytokinesis defects whereas deletion of fzlC causes synthetic defects with cell wall hydrolysis factors. Our characterization of FzlC as a novel membrane anchor for FtsZ expands our understanding of FtsZ regulators and establishes a role for membrane-anchored FtsZ in the control of cell wall hydrolysis. We also investigated the membrane anchoring function of the broadly conserved complex, FtsEX, and demonstrate that in C. crescentus FtsEX relays signals from the cytoplasm to the cell wall to regulate key developmental shape changes. Consistent with studies in diverse bacteria, we observe strong synthetic interactions between ftsE and cell wall hydrolytic factors, suggesting that regulation of cell wall remodeling is a conserved function of FtsEX. Intriguingly, without FtsE, cells frequently fail to separate and instead elaborate a thin, tubular structure between cell bodies, a growth mode observed in other α-proteobacteria. Overall, our results highlight the plasticity of bacterial cell shape and demonstrate how altering the activity of one morphogenetic program can produce diverse morphologies resembling those of other bacteria in nature.