Isolation and analysis of new genes that regulate the actin cytoskeleton system

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The actin cytoskeleton system plays an important role in cell motility, cell-cell adhesion, formation of cell polarity, and cell morphological change. Type I myosins are actindependent motor proteins and are involved in the reorganization of the actin cytoskeleton and endocytosis. Myo3p and Myo5p are the type I myosins in the budding yeast Saccharomyces cerevisiae. In this study, we have isolated a new gene, CDC50, as a multicopy suppressor of the myo5 mutant. The cdc50 mutant was first characterized as a mutant which show cell division cycle arrest, but its functions has not been characterized yet. CDC50 encodes a putative membrane protein that possesses a transmembrane domain. Biochemical experiments indicate that Cdc50p is an integral membrane protein. Microscopic observation of cells expressing CDC50-GFP indicates that CDC50 is localized to endosomes, suggesting that CDC50 is involved in the protein or lipid traffic through endosomes. The cdc50 mutant shows a coldsensitive growth phenotype and cells are arrested at small-budded stage, suggesting that Cdc50p regulates cell polarization. Visualization of the actin cytoskeleton in the cdc50 mutant revealed that actin patches are delocalized in the cdc50 mutant. Our results indicate that CDC50 regulates a traffic to the plasma membrane of a protein that is involved in the polarization of the actin cytoskeleton.