FtsZ CTL regulates polymer structure and dynamics

The bacterial tubulin FtsZ polymerizes to form a discontinuous cytokinetic ring that drives bacterial cell division by directing local cell wall synthesis. FtsZ comprises a polymerizing GTPase domain, an intrinsically disordered C-terminal linker (CTL) and a C-terminal conserved α-helix (CTC). FtsZ protofilaments align circumferentially in the cell, with the CTC mediating attachment to membrane-associated division proteins. The dynamic turnover and treadmilling of clusters of FtsZ protofilaments guides cell wall synthesis and constriction. The nature and regulation of the interactions that result in the assembly of protofilaments into dynamic clusters is unknown. Here, we describe a role for the CTL of Caulobacter crescentus FtsZ as an intrinsic regulator of lateral interactions between protofilaments in vitro. FtsZ lacking its CTL (∆CTL) shows dramatically increased propensity to form long multifilament bundles compared to wildtype (WT). ∆CTL has reduced GTP hydrolysis rate compared to WT. However, reducing protofilament turnover in WT is not sufficient to induce bundling. Surprisingly, binding of the membrane-anchoring protein FzlC disrupts ∆CTL bundling in a CTC-dependent manner. Moreover, the CTL affects the ability of FtsZ curving protein FzlA to promote formation of helical bundles. We conclude that the CTL of FtsZ influences polymer structure and dynamics both through intrinsic effects on lateral interactions and turnover and by influencing extrinsic regulation of FtsZ by binding partners. Our characterization of CTL function provides a biochemical handle for understanding the relationship between Z-ring structure and function in bacterial cytokinesis. Canonical cytoskeletal proteins in animal cells polymerize to form structural elements that provide shape and mechanical integrity to the cell. In bacteria, however, the cell wall is the primary structural element, maintaining cell shape and preventing lysis. The role of bacterial cytoskeletal proteins that impact cell shape is in the spatial and temporal regulation of cell wall synthesis (1, 2). Cytoskeletal polymer assembly, structure, and dynamics collectively regulate local shape changes by constraining and/or directing cell wall remodeling enzymes (2). During cell division, the cytoskeletal protein FtsZ polymerizes to form the cytokinetic ring or “Z-ring” at the incipient division site and recruits over two dozen proteins, including cell wall enzymes (3-5). The Z-ring comprises discontinuous clusters of circumferentially aligned FtsZ protofilaments that are highly dynamic (6-9). How protofilaments are arranged within the resolution-limited clusters and how they interact with each other is largely unknown. Recent studies in Escherichia coli and Bacillus subtilis demonstrated that FtsZ protofilament clusters in the Z-ring undergo treadmilling motion (8, 9). FtsZ treadmilling, in turn, drives the circumferential movement of cell wall enzymes, thereby directing local cell wall synthesis and remodeling towards constriction. The treadmilling of FtsZ clusters is presumably driven by the polymerization and . CC-BY 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/171280 doi: bioRxiv preprint first posted online Aug. 1, 2017;