Identification and Characterization of the Autophosphorylation Sites of Phosphoinositide 3-Kinase Isoforms β and γ

Abstract 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 IA PI3Kβ and p101-coupled class IB PI3Kγ 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 p110β and serine 1101 of p110γ). Like other class IA PI3K isoforms, autophosphorylation of p110β resulted in down-regulated PI3Kβ lipid kinase activity. However, no inhibitory effect of p110γ autophosphorylation on PI3Kγ lipid kinase activity was observed. Moreover, PI3Kβ and PI3Kγ differed in the regulation of their autophosphorylation. Whereas p110β autophosphorylation was stimulated neither by Gβγ complexes nor by a phosphotyrosyl peptide derived from the platelet-derived growth factor receptor, autophosphorylation of p110γ was significantly enhanced by Gβγ in a time- and concentration-dependent manner. In summary, we show that autophosphorylation of both PI3Kβ and PI3Kγ 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 PI3Kβ and PI3Kγ.