Steric hindrance in the upper 50 kDa domain of the motor Myo2p leads to cytokinesis defects in fission yeast

Cytokinesis in many eukaryotes requires a contractile actomyosin ring that is placed at the division site. In fission yeast, which is an attractive organism for the study of cytokinesis, actomyosin ring assembly and contraction requires the myosin II heavy chain Myo2p. Although -E1, a temperature-sensitive mutant defective in the upper 50 kDa domain of Myo2p, has been studied extensively, the molecular basis of the cytokinesis defect is not understood. Here, we isolate -E1-Sup2, an intragenic suppressor that contains the original mutation in -E1 (G345R) and a second mutation in the upper 50 kDa domain (Y297C). Unlike -E1-Sup1, a previously characterized -E1 suppressor, -E1-Sup2 reverses actomyosin ring contraction defects and Structural analysis of available myosin motor domain conformations suggests that a steric clash in -E1, which is caused by the replacement of a glycine with a bulky arginine, is relieved in -E1-Sup2 by mutation of a tyrosine to a smaller cysteine. Our work provides insight into the function of the upper 50 kDa domain of Myo2p, informs a molecular basis for the cytokinesis defect in -E1, and may be relevant to the understanding of certain cardiomyopathies.