Lateral interactions override nucleotide state in determining FtsZ filament curvature

Abstract

The bacterial tubulin homolog FtsZ assembles into dynamic filaments that form the cytokinetic Z-ring and drives constriction during cell division. Whether a nucleotide-dependent FtsZ filament curvature plays a role in constriction is often debated. Here, we combine cryo-electron microscopy and molecular dynamics simulations to understand the structural basis of FtsZ filament curvature. Cryo-EM structures of GTP-bound Spiroplasma FtsZ filaments in two curved states emphasize that curvature is an intrinsic property of FtsZ filament, confirming recent models in which GTP hydrolysis does not dictate protofilament bending in the tubulin family. Consistently, molecular dynamics simulations demonstrate that GTP-bound filaments can adopt a range of curved conformations. The preferred intrinsic curvature appears to be such that the C-terminal end of the globular domain faces the convex surface. Structural analyses of the curved conformations identify dynamic and stationary zones at the longitudinal interfaces of the protofilament, suggesting that structural plasticity of the intermonomer interface contributes to filament bending. Furthermore, we demonstrate that lateral interactions between adjacent protofilaments straighten the filaments, overriding their relaxed curved states. Optimal orientations of lateral interactions in the Z-ring assembly could be brought about by other interacting proteins of the divisome machinery. The straighter filament conformation is likely to stimulate a higher GTPase activity. Together, our findings establish lateral association as a primary determinant for straight FtsZ filaments, analogous to the tubulin protofilaments in a microtubule lattice. The snapshots of structural states provide a mechanistic basis for how the intrinsic curvature facilitates association on the membrane and the physiological relevance of transitions between bent and straight conformations of the FtsZ filament during Z-ring assembly and constriction.