An EGF receptor/Ral-GTPase signaling cascade regulates c-Src activity and substrate specificity

c-Src is a membrane-associated tyrosine kinase that can be activated by man y types of extracellular signals, and can regulate the function of a variet y of cellular protein substrates. We demonstrate that epidermal growth fact or (EGF) and P-adrenergic receptors activate c-Src by different mechanisms leading to the phosphorylation of distinct sets of c-Src substrates. In par ticular, we found that EGF receptors, but not beta(2)-adrenergic receptors, activated c-Src by a Ral-GTPase-dependent mechanism. Also, c-Src activated by EGF treatment or expression of constitutively activated Ral-GTPase led to tyrosine phosphorylation of Stat3 and cortactin, but not Shc or subseque nt Erk activation. In contrast, c-Src activated by isoproterenol led to tyr osine phosphorylation of Shc and subsequent Erk activation, but not tyrosin e phosphorylation of cortactin or Stat3. These results identify a role for Ral-GTPases in the activation of c-Src by EGF receptors and the coupling of EGF to transcription through Stat3 and the actin cytoskeleton through cort actin, They also show that c-Src kinase activity can be used differently by individual extracellular stimuli, possibly contributing to their ability t o generate unique cellular responses.