Actin turnover protects the cytokinetic contractile ring from structural instability

Abstract

In common with other actomyosin contractile cellular machineries, actin turnover is required for normal function of the cytokinetic contractile ring. Cofilin is an actin-binding protein contributing to turnover by severing actin filaments, required for cytokinesis by many organisms. In fission yeast cofilin mutants, contractile rings suffer bridging instabilities in which actin bundles peel away from the plasma membrane into straight bridges. The origin of this behaviour is unclear. Here we used molecularly explicit simulations of the fission yeast contractile ring to examine the role of cofilin. Simulations reproduced the experimentally observed cycles of bridging and reassembly during constriction, each lasting [~] 6 min, and the tendency for bridging to occur in ring segments with low myosin II Myo2 density. The lack of cofilin severing produced [~] 2-fold longer filaments and, consequently, [~] 2-fold higher ring tensions. Simulations identified bridging as originating in the boosted ring tension, which increased centripetal forces that detached actin from Myo2 that anchored actin to the membrane. Thus, cofilin serves a critical role in cytokinesis by protecting the contractile ring from bridging, the principal structural threat. Summary statementMolecularly explicit simulations showed that cofilin-mediated actin severing protects the fission yeast cytokinetic contractile ring from instabilities in which actin peels away into straight bridges.