NERVE GROWTH-FACTOR INDUCED STIMULATION OF RAS REQUIRES TRK INTERACTION WITH SHC BUT DOES NOT INVOLVE PHOSPHOINOSITIDE 3-OH KINASE

The TrkA receptor protein tyrosine kinase is involved in signalling PC 12 cell differentiation and cessation of cell division in response to nerve growth factor (NGF), To assess the importance of adaptor protein s and Pas in NGF control of phosphoinositide 3-OH kinase (PI 3-kinase) , specific receptor mutations in Trk have been employed.. We show that phosphorylation of tyrosine 490, but not 785, of Trk is essential for activation of both Ras and PI 3-kinase in vivo, correlating with tyro sine phosphorylation of Shc and binding of Shc to the adaptor Grb2 and the Pas exchange factor Sos. A mutant receptor that lacks Y490 and Y7 85, but contains an introduced YxxM motif which binds the regulatory d omain of PI 3-kinase, is unable to activate Pas despite causing increa sed PI 3-kinase activity. This indicates clearly that activation of PI 3-kinase by itself is not sufficient to cause activation of Pas, argu ing against a model in which PI 3-kinase acts upstream of Pas, The Shc site of Trk is thus crucial for the activation of Pas and PI 3-kinase .