TYROSINE PHOSPHORYLATION OF G-PROTEIN ALPHA-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-src kinase family and those mediated by guanine nucleotide regulatory proteins (G proteins). In this study, we explore the possibility that direct interactions between pp60c-src and G proteins may occur with functional consequences. Preparations of pp60c-src isolated by immunoprecipitation phosphorylate on tyrosine residues the purified G-protein alpha-subunits (G-alpha) of several heterotrimeric G proteins. Phosphorylation is highly dependent on G-protein conformation, and G-alpha(GDP) uncomplexed by beta-gamma-subunits appears to be the preferred substrate. In functional studies, phosphorylation of stimulatory Ga (G-alpha(s)) modestly increases the rate of binding of guanosine 5'-[gamma-[S-35]thio]triphosphate to G(s) as well as the receptor-stimulated steady-state rate of GTP hydrolysis by G(s). Heterotrimeric G proteins may represent a previously unappreciated class of potential substrates for pp60c-src.