Charge segregation in the C-terminal linker of FtsZ enables Z-ring scaling with prokaryotic cell width

Bacterial cytokinesis is orchestrated by the Z-ring, a cytoskeletal structure formed by treadmilling filaments of the tubulin-like GTPase FtsZ. During assembly, filament curvature must match the cell diameter, yet how curvature is encoded in FtsZ remains unclear. Using in vitro reconstitution with purified proteins and supported lipid bilayers, we identify FtsZs intrinsically disordered C-terminal linker (CTL) as a curvature tuner that determines the Z-ring diameter. Increasing CTL charge segregation ({kappa}) progressively reduces Z-ring diameter without altering treadmilling dynamics, monomer turnover, or GTPase activity, indicating that {kappa} modulates filament curvature rather than polymerization kinetics. Linker-dilution experiments, in which wild-type FtsZ is mixed with a CTL-deletion mutant ({Delta}CTL), further show that inter-CTL electrostatic interactions straighten filaments, while weakening these contacts increases curvature and compacts rings. To test whether this mechanism is conserved, we analyzed 4,603 prokaryotic FtsZ sequences and found that CTL charge segregation quantified by {kappa} - but not CTL length, fraction of charged residues, or net charge per residue - correlates with cell width: at low {kappa}, cells show a broad range of widths, whereas high {kappa} values exclude wide cells. Co-expression of {Delta}CTL variants in vivo perturbs Z-ring positioning and impairs growth, consistent with a curvature mismatch and defects during assembly. Together, these results suggest a simple evolutionary mechanism in which CTL charge patterning encodes intrinsic filament curvature, enabling Z-ring geometry to scale with cell diameter and linking protein sequences to bacterial morphology.