Induction of Rac-guanine nucleotide exchange activity of Ras-GRF1/CDC25Mm following phosphorylation by the nonreceptor tyrosine kinase Src

Ras-GRF1/CDC25(Mm) has been implicated as a Ras-guanine nucleotide exchange factor (GEF) expressed in brain. Ras-GEF activity of Ras-GRF1 is augmented in response to Ca2+ influx and G protein beta gamma subunit (G beta gamma) stimulation. Ras-GRF1 also acts as a GEF toward Rac, but not Rho and Cdc42, when activated by G beta gamma-mediated signals, Tyrosine phosphorylation of Ras-GRF1 is critical for the induction of Rac-GEF activity as evidenced by inhibition by tyrosine kinase inhibitors. Herein, we show that the nonreceptor tyrosine kinase Src phosphorylates Ras-GRF1, thereby inducing Rac-GEF activity. Ras-GRF1 transiently expressed with v-Src was tyrosine-phosphorylated and showed significant GEF activity toward Rac, but not Rho and Cdc42, which was comparable with that induced by G beta gamma. In contrast, Ras-GEF activity remained unchanged. The recombinant c-Src protein phosphorylated affinity-purified glutathione S-transferase tagged Ras-GRF1 in vitro and thereby elicited Rac-GEF activity. Taken together, tyrosine phosphorylation by Src is sufficient for the induction of Rac-GEF activity of Ras-GRF1, which may imply the involvement of Src downstream of G beta gamma to regulate Ras-GRF1.