Regulation of phosphatidylinositol 4-phosphate 5-kinase from Schizosaccharomyces pombe by casein kinase I

Phosphatidylinositol (4)P 5-kinase (PtdIns(4)P B-kinase) catalyzes the last step in the synthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5 )P-2). PtdIns(4,B)P-2 is a precursor of diacylglycerol and inositol 1,45-tr isphosphate and is also involved in regulation of actin cytoskeleton remode ling and membrane traffic. To satisfy such varied demands in several aspect s of cell physiology, synthesis of PdIns(4,5)P-2 must be stringently regula ted. In this paper we describe extraction, purification, and characterizati on of PtdIns(4)P 5-kinase from the plasma membranes of Schizosaccharomyces pombe. We also provide evidence that PtdIns(4)P 5-kinase is phosphorylated and inactivated by Cki1, the S. pombe homolog of casein kinase I. Phosphory lation by Cki1 in vitro decreases the activity of PtdIns(4)P 5-kinase. In a ddition, and most importantly, overexpression of Cki1 in S. pombe results i n a reduced synthesis of PtdIns(4,5)P-2 and in a lower activity of PtdIns(4 )P 5-kinase associated with the plasma membrane, These results suggest that PtdIns(4)P 5-kinase is a target of Cki1 in S. pombe and that Cki1 is invol ved in regulation of PtdIns(4, 5)P-2 synthesis by phosphorylating and inact ivating PtdIns(4)P 5-kinase.