Myosins Generate Contractile Force and Maintain Organization in the Fission Yeast Cytokinetic Contractile Ring

During cytokinesis, cells assemble an actomyosin contractile ring whose tension constricts and divides cells, but the ring tension has rarely been measured. Actomyosin force generation is well understood for the regular sarcomeric architecture of striated muscle, but super-resolution studies of fission yeast contractile rings revealed organizational building blocks that are not sarcomeres but irregularly positioned plasma membrane-anchored protein complexes called nodes. Here we measured contractile ring tensions in fission yeast protoplast cells. The myosin II isoforms Myo2 and Myp2 generated the tension, with a ~2-fold greater contribution from Myo2. Simulations of a molecularly detailed ring model revealed a sliding node mechanism for tension, where nodes hosting tense actin filaments were pulled bidirectionally around the ring. Myo2 and Myp2 guided self-assembling components into the ring organization, and anchored the ring against bridging instabilities. Thus, beyond force production, Myo2 and Myp2 are the principal organizers, bundlers and anchors of the contractile ring.