MSS4, A PHOSPHATIDYLINOSITOL-4-PHOSPHATE 5-KINASE REQUIRED FOR ORGANIZATION OF THE ACTIN CYTOSKELETON IN SACCHAROMYCES-CEREVISIAE

The Saccharomyces cerevisiae protein MSS4 is essential and homologous to mammalian phosphatidylinositol-4-phosphate (PI(4)P) 5-kinases. Here , we demonstrate that MSS4 is a lipid kinase. MSS4 has dual substrate specificity in vitro, converting PI(4)P to PI(4,5)P-2 and to a lesser extent PI(S)P to PI(3, 4)P-2; no activity was detected with PI or PI(S )P as a substrate. Cells overexpressing MSS4 contain an elevated level specifically of PI(4,5)P,, whereas mss4 mutant cells have only approx imately 10% of the normal amount of this phosphorylated phosphoinositi de. Furthermore, cells lacking MSS4 are unable to form actin cables an d to properly localize their actin cytoskeleton during polarized cell growth. Overexpression of RHO2, encoding a Rho-type GTPase involved in regulation of the actin cytoskeleton, restores growth and polarized d istribution of actin in an mss4 mutant. These results suggest that MSS 4 is the major PI(4)P B-kinase in yeast and provide a link between pho sphoinositide metabolism and organization of the actin cytoskeleton in vivo.