Structure and Constriction Mechanism of the Actomyosin Ring

Cytokinesis is orchestrated by a contractile actomyosin ring, but its structure and mechanism remain elusive. We visualized the 3D structure of the ring in frozen-hydrated dividing yeast cells by electron cryotomography (ECT). Detailed arrangements of actin filaments within the ring and with respect to the membrane were seen for the first time, providing a crucial spatial constraint for the constriction mechanism of the ring. Using the ECT data and input from the literature we then explored sixteen mechanistic models by coarse-grained simulations at the 3D molecular details, revealing plausible mechanisms for preventing membrane distortion and protein aggregation. We found that, in the model that best fits experimental data, both bipolar and membrane-attached unipolar myosins exist in the ring, reconciling two different views in the field regarding the myosin configuration. In this model, ring tension is generated primarily by interactions between bipolar myosins and actin, and transmitted to the membrane via unipolar myosins. This model recapitulates a broad distribution of distances from actin filaments to the membrane observed in our tomograms and separation of two different myosin isoforms into the outer and inner subdomains of the ring reported in a previous fluorescence microscopy study. Further, it rationalizes how bundles of actomyosin were able to separate from the membrane in fluorescence microscopy experiments of the same previous study