TYPE-II MYOSIN INVOLVED IN CYTOKINESIS IN THE FISSION YEAST, SCHIZOSACCHAROMYCES-POMBE

We have cloned an unique gene encoding the heavy chain of a type II my osin in the fission yeast, Schizosaccharomyces pombe. The myo2(+) gene encodes a protein of 1526 amino acids with a predicted molecular weig ht of 177 kDa and containing consensus binding motifs for both essenti al and regulatory light chains. The S. pombe myo2(+) head domain is 45 % identical to myosin IIs from Saccharomyces cerevisiae and Homo sapie ns and 40% identical to Drosophila melanogaster. Structurally, myo2(+) most closely resembles budding yeast MYO1, the tails of both myosin I Is containing a number of proline residues that are predicted to subst antially disrupt the ability of these myosins to form coiled coils. Th e myo2(+) gene is located on chromosome III, 8.3 map units from ade6(). Deletion of approximately 70% of the coding sequence of myo2(+) is lethal but myo2 Delta spores can acquire a suppressor mutation that al lows them to form viable microcolonies consisting of filaments of bran ched cells with aberrant septa. Overexpression of myo2(+) results in t he inhibition of cytokinesis; cells become elongated and multinucleate and fail to assemble a functional cytokinetic actin ring and are eith er aseptate or form aberrant septa. These results suggest that a contr actile actin-myosin based cytokinetic mechanism appeared early in the evolution of eukaryotic cells and further emphasise the utility of fis sion yeast as a model organism in which to study the molecular and cel lular basis of cytokinesis. (C) 1997 Wiley-Liss, Inc.