Phosphatidylinositol 3-kinase: Inhibition of intrinsic protein-serine kinase activity by phosphoinositides, and of lipid kinase activity by Mn 2+

Abstract

Phosphatidylinositol (PI) 3-kinase is composed of 110 kDa catalytic and 85 kDa regulatory subunits. The 110 kDa subunit has two intrinsic kinase activities, i.e., Mn 2+-dependent protein-serine kinase and Mg 2+-dependent lipid kinase activities. These intrinsic kinases have been reported to be interdependent: protein-serine kinase phosphorylates the 85 kDa subunit of PI 3-kinase, which upon phosphorylation inhibits the lipid kinase activity of PI 3-kinase. We report here that phosphoinositides can selectively inhibit the protein-serine kinase activity of PI 3-kinase without affecting lipid kinase activity. This inhibition depends on the phosphorylation status of the phosphoinositides, i.e., PI 4,5-bisphosphate > PI 4-phosphate >>PI. Mn 2+ (2 mM) protected protein kinase activity from phosphoinositides-mediated inhibition if added prior to interaction of PI 3-kinase with phosphoinositides. On the other hand, Mn 2+ (2 mM) inhibited lipid kinase activity independent of its effect on the protein kinase activity of PI 3-kinase. The present study suggests that the protein-serine kinase and the lipid kinase activities of PI 3-kinase can be selectively inhibited by phosphoinositides and Mn 2+ respectively.