Expression and characterization of the p85 subunit of the phosphatidylinositol 3-kinase complex and a related p85β protein by using the baculovirus expression system

PtdIns 3-kinase associates with certain activated protein-tyrosine kinase receptors and with the pp60c-src/polyoma middle-T complex, suggesting that the enzyme is involved in growth regulation. The purified PtdIns 3-kinase appears to have two subunits, of 85 kDa and 110 kDa. Structural analysis at protein and cDNA levels revealed two forms of the 85 kDa subunit, one which associates with PtdIns 3-kinase activity termed p85 alpha, and a protein of unknown function, p85 beta. Both 85 kDa proteins contain src-homology regions 2 and 3 (SH2 and SH3), but lack enzymic activity, suggesting that they may be regulatory subunits of PtdIns 3-kinase. To probe their structure and function further, p85 alpha and p85 beta have been expressed and purified in large amounts from insect cells by using baculovirus vectors. Specific antisera detect p85 alpha, but not p85 beta, associated with PtdIns 3-kinase activity in various cell types. Co-expression studies in insect cells have shown that p85 alpha and p85 beta are substrates for the protein-tyrosine kinases of epidermal growth factor, colony-stimulating factor 1 and c-erbB2 receptors and the src family kinase p59c-fyn. Both p85 alpha and p85 beta form tight complexes with these protein-tyrosine kinases as measured by immunoprecipitation and kinase assays in vitro. The specificity of binding of free p85 is less restricted than that of p85 in the active PtdIns 3-kinase complex with the 110 kDa protein. The relevance of these results to growth-factor-induced PtdIns 3-kinase activation is discussed.