PI-3-KINASE IS A DUAL-SPECIFICITY ENZYME - AUTOREGULATION BY AN INTRINSIC PROTEIN-SERINE KINASE-ACTIVITY

Phosphatidylinositol 3-kinase (PI 3-kinase) has a regulatory 85 kDa ad aptor subunit whose SH2 domains bind phosphotyrosine in specific recog nition motifs, and a catalytic 110 kDa subunit. Mutagenesis of the p11 0 subunit, within a sequence motif common to both protein and lipid ki nases, demonstrates a novel intrinsic protein kinase activity which ph osphorylates the p85 subunit on serine at a stoichiometry of approxima tely 1 mol of phosphate per mol of p85. This protein-serine kinase act ivity is detectable only upon high affinity binding of the p110 subuni t with its unique substrate, the p85 subunit. Tryptic phosphopeptide m apping revealed that the same major peptide was phosphorylated in p85a lpha both in vivo in cultured cells and in the purified recombinant en zyme. N-terminal sequence and mass analyses were used to identify Ser6 08 as the major phosphorylation site on p85alpha. Phosphorylation of t he p85 subunit at this serine causes an 80% decrease in PI 3-kinase ac tivity, which can subsequently be reversed upon treatment with protein phosphatase 2A. These results have implications for the role of inter -subunit serine phosphorylation in the regulation of the PI 3-kinase i n vivo.