Motor Activity Dependent and Independent Functions of Myosin II Contribute to Actomyosin Ring Assembly and Contraction in Schizosaccharomyces pombe.

Cytokinesis depends on a contractile actomyosin ring in many eukaryotes [1-3]. Myosin II is a key component of the actomyosin ring, although whether it functions as a motor or as an actin cross-linker to exert its essential role is disputed [1, 4, 5]. In Schizosaccharomyces pombe, the myo2-E1 mutation affects the upper 50 kDa sub-domain of the myosin II heavy chain, and cells carrying this lethal mutation are defective in actomyosin ring assembly at the non-permissive temperature [6, 7]. myo2-E1 also affects actomyosin ring contraction when rings isolated from permissive temperature-grown cells are incubated with ATP [8]. Here we report isolation of a compensatory suppressor mutation in the lower 50 kDa sub-domain (myo2-E1-Sup1) that reverses the inability of myo2-E1 to form colonies at the restrictive temperature. myo2-E1-Sup1 is capable of assembling normal actomyosin rings, although rings isolated from myo2-E1-Sup1 are defective in ATP-dependent contraction in vitro. Furthermore, the product of myo2-E1-Sup1 does not translocate actin filaments in motility assays in vitro. Superimposition of myo2-E1 and myo2-E1-Sup1 on available rigor and blebbistatin-bound myosin II structures suggests that myo2-E1-Sup1 may represent a novel actin translocation-defective allele. Actomyosin ring contraction and viability of myo2-E1-Sup1 cells depend on the late cytokinetic S. pombe myosin II isoform, Myp2p, a non-essential protein that is normally dispensable for actomyosin ring assembly and contraction. Our work reveals that Myo2p may function in two different and essential modes during cytokinesis: a motor activity-independent form that can promote actomyosin ring assembly and a motor activity-dependent form that supports ring contraction.