SYNTHETIC LETHALITY SCREEN IDENTIFIES A NOVEL YEAST MYOSIN-I GENE (MYO5) - MYOSIN-I PROTEINS ARE REQUIRED FOR POLARIZATION OF THE ACTIN CYTOSKELETON

The organization of the actin cytoskeleton plays a critical role in ce ll physiology in motile and nonmotile organisms. Nonetheless, the func tion of the actin based motor molecules, members of the myosin superfa mily, is not well understood. Deletion of MYO3, a yeast gene encoding a ''classic'' myosin I, has no detectable phenotype. We used a synthet ic lethality screen to uncover genes whose functions might overlap wit h those of MYO3 and identified a second yeast myosin I gene, MYO5. MYO 5 shows 86 and 62% identity to MYO3 across the motor and non-motor reg ions. Both genes contain an amino terminal motor domain, a neck region containing two IQ motifs, and a tail domain consisting of a positivel y charged region, a proline-rich region containing sequences implicate d in ATP-insensitive actin binding, and an SH3 domain. Although myo5 d eletion mutants have no detectable phenotype, yeast strains deleted fo r both MYO3 and MYO5 have severe defects in growth and actin cytoskele tal organization. Double deletion mutants also display phenotypes asso ciated with actin disorganization including accumulation of intracellu lar membranes and vesicles, cell rounding, random bud site selection, sensitivity to high osmotic strength, and low pH as well as defects in chitin and cell wall deposition, invertase secretion, and fluid phase endocytosis. Indirect immunofluorescence studies using epitope-tagged Myo5p indicate that Myo5p is localized at actin patches. These result s indicate that MYO3 and MYO5 encode classical myosin I proteins with overlapping functions and suggest a role for Myo3p and Myo5p in organi zation of the actin cytoskeleton of Saccharomyces cerevisiae.