Ras activation mechanism. The dimerization of the EGF receptor due to EGF binding leads to: (1) the phosphorylation of the C-terminal tails of the receptor, (2) the subsequent recruitment of Grb2 and Sos, and (3) the exchange of GTP for GDP in Ras. This signal-transduction pathway results in the conversion of Ras into its activated GTP-bound form.