RAS IS ESSENTIAL FOR NERVE GROWTH FACTOR-INDUCED AND PHORBOL ESTER-INDUCED TYROSINE PHOSPHORYLATION OF MAP KINASES

Treatment of PC12 cells with nerve growth factor (NGF) induces a rapid increase in tyrosine phosphorylation of multiple cellular proteins. Expression of a dominant inhibitory Ras mutant specifically blocked NGF- and TPA-induced tyrosine phosphorylation of two proteins of approximately 42 and 44 kd. Conversely, expression of an oncogenic variant of Ras induced tyrosine phosphorylation of the same 42 and 44 kd proteins. The 44 kd protein was immunoprecipitated with an antibody directed against extracellular signal-regulated kinase 1/mitogen-activated protein kinase (MAPK) and the 42 kd protein comigrated with a 42 kd MAPK, indicating that at least one and probably both Ras-regulated phosphoproteins are MAPKs. In addition, MAPK activation, as measured by in vitro phosphorylation of myelin basic protein, was also regulated by Ras. Ras was not required for NGF-induced activation of Trk or tyrosine phosphorylation of PLC-gamma-1. Thus, NGF-induced tyrosine phosphorylation occurs both prior to and following Ras action, and Ras plays a critical role in the NGF- and TPA-induced tyrosine phosphorylation of MAPKs.