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 many types of extracellular signals, and can regulate the function of a variety of cellular protein substrates. We demonstrate that epidermal growth factor (EGF) and β‐adrenergic receptors activate c‐Src by different mechanisms leading to the phosphorylation of distinct sets of c‐Src substrates. In particular, we found that EGF receptors, but not β 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 subsequent Erk activation. In contrast, c‐Src activated by isoproterenol led to tyrosine phosphorylation of Shc and subsequent Erk activation, but not tyrosine 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 cortactin. They also show that c‐Src kinase activity can be used differently by individual extracellular stimuli, possibly contributing to their ability to generate unique cellular responses.