Actin-associated proteins in the yeast Saccharomyces cerevisiae.

The MYO1 gene encodes the type II myosin heavy chain in the yeast Saccharomyces cerevisiae. The 5' end of the MYO1 gene was deleted to produce a myol null mutant (myo1). The resulting mutation was not lethal and cells displayed a phenotype that was indistinguishable from cells bearing a disruption in the 3' end of the gene, ?myol mutants are defective in cytokinesis, resulting in long chains of cells. Although nuclear division proceeds, there is a defect in nuclear migration resulting in cells containing multiple nuclei. The myo1 mutant tends to lyse when grown in liquid medium and was found to be sensitive to changes in pH. The mutant also showed defects in vacuolar morphology, chitin distribution and apparent defects in bud site selection. An attempt was made to identify other myosin heavy chain proteins by purifying actomyosin complexes. A 200kDa protein was co-purified with the myosin heavy chain following two rounds of purification. Polyclonal antibodies were raised against the purified protein and used to screen a yeast expression library. A single clone was isolated and identified as the yeast FAS1 gene. FAS1 encodes the beta-subunit of the yeast fatty acid synthetase. The disruption of the FAS1 gene confirmed that fatty acid synthetase was a component of actomyosin preparations and offers the potential for purifying large amounts of myosin heavy chain from yeast.