Interaction between Src and a C-terminal proline-rich motif of Akt is required for Akt activation

Activation of Akt by growth factors is a multistep process. Here, we provide evidence that tyrosine kinase Src is directly associated with Akt through the interaction between its SH3 domain and a conserved proline-rich motif (PXXP) in the C-terminal regulatory region of Akt. Substitution of the proline residues Pro-424 and Pro-427 by alanines results in loss of Akt activity and phosphorylation induced by the epidermal growth factor (EGF), possibly because these mutations disrupt the interaction between Akt and the SH3 domain of Src. This possibility is corroborated by our observation that the Akt mutant lacking these two prolines fails to bind to Src both in vivo and in vitro. We also showed that phosphorylation of Tyr-315 in Akt induced by Src or EGF is dependent on the integrity of this proline-rich motif. Furthermore, the Akt mutant lacking this proline motif fails to block the transcription activity of Forkhead in 293 cells and poorly stimulates the proliferation of Madin-Darby canine kidney cells. Taken together, our data suggest that the interaction between the SH3 domain of Src family kinases and the proline-rich motif in the C-terminal regulatory region of Akt is required for tyrosine phosphorylation of Akt and its subsequent activation. It is noteworthy that this PXXP motif is conserved throughout several members of AGC kinase family, implying that association of this motif with the SH3 domain of an upstream regulator may represent a general mechanism applicable to these kinases as well.