Phosphoinositides regulate a wide range of cellular processes, including proliferation, survival, cytoskeleton remodelling and membrane trafficking, yet the mechanisms controlling the kinases, phosphatases and lipases that modulate phosphoinositide levels are poorly understood. In the present study, we describe a mechanism controlling MSS4, the sole phosphatidylinositol (4)-phosphate 5-kinase in Saccharomyces cerevisiae. Mutations in MSS4 and CMD1, encoding the small Ca2+-binding protein calmodulin, confer similar phenotypes, including loss of viability and defects in endocytosis and in organization of the actin cytoskeleton. Overexpression of MSS4 suppresses the growth and actin defects of cmd1-226, a temperature-sensitive calmodulin mutant which is defective in the organization of the actin cytoskeleton. Finally, the cmd1-226 mutant exhibits reduced levels of phosphatidylinositol (4,5)-bisphosphate. These findings suggest that calmodulin positively controls MSS4 activity and thereby the actin cytoskeleton.

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