Kovalenko, I. B. | Search bioRxiv

bioRxiv2026

Lateral interactions override nucleotide state in determining FtsZ filament curvature

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.

biochemistry↗

bioRxiv2025

Control of the force-bearing properties of microtubule-associated proteins via stalk/linker regions: insights from the NDC80 complex

Many microtubule-associated proteins (MAPs) function under mechanical loads. Among them, motor proteins and passive couplers link microtubules with other cytoskeletal filaments, membranous structures and diverse scaffolds to enable cell shape changes, locomotion and other important processes. A key kinetochore complex, NDC80, transmits forces from microtubule disassembly to chromosome motion during cell division. Recently, this complex has been shown to detach from microtubules more easily when pulled toward the minus-end of the microtubule than when pulled in the plus-end direction. Here, we used coarse-grained molecular dynamics and Brownian dynamics simulations to explain the asymmetric effect of the directional load on the unbinding of the NDC80 complex from microtubules and then generalized our findings to other MAPs. We found that the lever arm created by the stiff stalk of NDC80 tilted toward the plus-end of the microtubule is critical for asymmetric unbinding of this complex, similar to that of dynein. In contrast, EB-proteins, the microtubule crosslinker PRC1, and kinesins are predicted to lack pronounced unbinding asymmetry, either due to their almost perpendicular anchorage to the microtubule wall or due to the high flexibility of their linker regions proximal to the microtubule-binding domains. Thus, our study highlights some of the design principles of MAPs, explaining how their distal parts can impart, modulate or eliminate the dependence of unbinding on the direction of external loads. This information deepens our understanding of the load-bearing properties and functions of diverse MAPs and may guide the design of synthetic protein systems with predefined mechanical characteristics.

biophysics↗