Identification and characterization of the autophosphorylation sites of phosphoinositide 3-kinase isoforms β and γ

Class I phosphoinositide 3-kinases (PI3Ks) are bifunctional enzymes possessing lipid kinase activity and the capacity to phosphorylate their catalytic and/or regulatory subunits. In this study, in vitro autophosphorylation of the G protein-sensitive p85-coupled class I-A PI3Kbeta and p101-coupled class I-B PI3Kgamma was examined. Autophosphorylation sites of both PI3K isoforms were mapped to C-terminal serine residues of the catalytic p110 subunit (i.e. serine 1070 of p110beta and serine 1101 of p110gamma). Like other class I-A PI3K isoforms, autophosphorylation of p110beta resulted in down-regulated PI3Kbeta lipid kinase activity. However, no inhibitory effect of p110gamma autophosphorylation on PI3Kgamma lipid kinase activity was observed. Moreover, PI3Kbeta and PI3Kgamma differed in the regulation of their autophosphorylation. Whereas p110beta autophosphorylation was stimulated neither by Gbetagamma complexes nor by a phosphotyrosyl peptide derived from the platelet-derived growth factor receptor, autophosphorylation of p110gamma was significantly enhanced by Gbetagamma in a time- and concentration-dependent manner. In summary, we show that autophosphorylation of both PI3Kbeta and PI3Kgamma occurs in a C-terminal region of the catalytic p110 subunit but differs in its regulation and possible functional consequences, suggesting distinct roles of autophosphorylation of PI3Kbeta and PI3Kgamma.