Tyrosine Phosphorylation of G Protein α Subunits by pp60c-src

A number of lines of evidence suggest that cross-talk exists between the cellular signal transduction pathways involving tyrosine phosphorylation catalyzed by members of the pp60c-srckinase family and those mediated by guanine nucleotide regulatory proteins (G proteins). In this study, we explore the possibility that direct interactions between pp60c-srcand G proteins may occur with functional consequences. Preparations of pp60c-srcisolated by immunoprecipitation phosphorylate on tyrosine residues the purified G-protein α subunits (Gα) of several heterotrimeric G proteins. Phosphorylation is highly dependent on G-protein conformation, and Gα(GDP) uncomplexed by βγ subunits appears to be the preferred substrate. In functional studies, phosphorylation of stimulatory Gα (Gαs) modestly increases the rate of binding of guanosine 5'-[γ-[35S]thio]triphosphate to Gsas well as the receptor-stimulated steady-state rate of GTP hydrolysis by Gs. Heterotrimeric G proteins may represent a previously unappreciated class of potential substrates for pp60c-src.