IDENTIFICATION OF MYO4, A 2ND CLASS-V MYOSIN GENE IN YEAST

We have isolated a fourth myosin gene (MY04) in yeast (Saccharomyces c erevisiae). MY04 encodes a similar to 170 kDa (1471 amino acid) class V myosin, using the classification devised by Cheney et al. (1993a; Ce ll Motil. Cytoskel. 24, 215-223); the motor domain is followed by a ne ck region containing six putative calmodulin-binding sites and a tail with a short potential 'coiled-coil' domain. A comparison with other m yosins in GenBank reveals that Myo4 protein is most closely related to the yeast Myo2 protein, another class V myosin. Deletion of MY04 prod uces no detectable phenotype, either alone or in conjunction with muta tions in myo2 or other myosin genes, the actin gene, or secretory gene s. However, overexpression of MY04 or MY02 results in several morpholo gical abnormalities, including the formation of short strings of unsep arated cells in diploid strains, or clusters of cells in haploid strai ns. Alterations of MY04 or MY02 indicate that neither the motor domain s nor tails of these myosins are required to confer the overexpression phenotype, whereas the neck region may be required. Although this phe notype is similar to that seen upon MYO1 deletion, we provide evidence that the overexpression of Myo4p or Myo2p is not simply interfering w ith Myo1p function.