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  myo2 -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  myo2 -E1-Sup2, an intragenic suppressor that contains the original mutation in  myo2 -E1 (G345R) and a second mutation in the upper 50 kDa domain (Y297C). Unlike  myo2 -E1-Sup1, a previously characterized  myo2 -E1 suppressor,  myo2 -E1-Sup2 reverses actomyosin ring contraction defects  in vitro  and  in vivo  Structural analysis of available myosin motor domain conformations suggests that a steric clash in  myo2 -E1, which is caused by the replacement of a glycine with a bulky arginine, is relieved in  myo2 -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  myo2 -E1, and may be relevant to the understanding of certain cardiomyopathies.
Palani S, Srinivasan R, Zambon P, Kamnev A, Gayathri P, Balasubramanian MK