IDENTIFICATION OF THE MITOGEN-ACTIVATED PROTEIN-KINASE PHOSPHORYLATION SITES ON HUMAN SOS1 THAT REGULATE INTERACTION WITH GRB2

The Son of sevenless proteins (Sos) are guanine nucleotide exchange fa ctors involved in the activation of Ras by cytoplasmic and receptor ty rosine kinases, Growth factor stimulation rapidly induces the phosphor ylation of Sos on multiple serine and threonine sites, Previous studie s have demonstrated that growth factor-induced Sos phosphorylation occ urs at the C-terminal region of the protein and is mediated, in part, by mitogen-activated protein (MAP) kinase. In this report, we describe the identification of five MAP kinase sites (S-1137, S-1167, S-117S, S-1193, and S-1197) on hSos1. We demonstrate that four of these sites, S-1132, S-1167, S-1175, and S-1193, become phosphorylated following g rowth factor stimulation, The MAP kinase phosphorylation sites are clu stered within a region encompassing three proline-rich SH3-binding sit es in the C-terminal domain of hSos1, Replacing the MAP kinase phospho rylation sites with alanine residues results in an increase in the bin ding affinity of Grb2 to hSos1. Interestingly, hSos2 contains only one MAP kinase phosphorylation site and, as demonstrated previously, has an increased affinity ton ard Grb2 compared with hSos1, These results suggest a role for MAP kinase in the regulation of Grb2-Sos interactio ns, Since the binding of Grb2 is important for Sos function, the phosp horylation-dependent modulation of Grb2-Sos association may provide a means of controlling Ras activation.