Some 50 years ago it was discovered that non-muscle myosin II generates the force that constricts the cytokinetic ring to divide cells. Here, we show an equally critical role for myosin II is to stabilize the structure of the actomyosin ring. Combining molecularly detailed simulations and experiments to study the fission yeast contractile ring, we find the type II myosin Myo2 (1) anchors the ring to the plasma membrane and (2) suppresses bridging, the principal instability threatening actomyosin ring architecture which, if uncontrolled, detaches entire sections from the membrane into straight bridges. Myo2 belongs to membrane-anchored complexes in the fission yeast ring called nodes. We systematically modified and/or shortened the Myo2 tail, which comprises two rigid coiled-coil domains and a flexible C-terminal region linking Myo2 to the nodes (Motegi et al., 2004). Constructs unable to bind nodes were inviable regardless of tail length but, remarkably, in both experiments and simulations ring integrity and functionality were restored when constructs were artificially re-tethered to alternative plasma membrane components. Thus, Myo2-mediated ring anchoring is required for function, rather than binding of myo2 to any specific partner. Structural stability of the ring required sufficiently long Myo2 reach. With sufficiently shortened Myo2 tails, the reach of actin-binding Myo2 head domains was insufficient to bundle enough actin filaments to maintain ring structure: bridging instabilities were activated, and in simulations ring tensions fluctuated wildly. For even shorter tails, ring tension declined catastrophically. Thus, beyond force production, Myo2 is a uniquely labile anchor whose motility allows it to anchor the highly dynamic actomyosin contractile ring, a task unsuited to static anchors which would rapidly detach. Myo2 stabilizes the entire ring architecture against ring-tension-generated structural instabilities by co-anchoring actin filaments across the actomyosin bundle cross-section.
Zhu et al. (Sun,) studied this question.