GROWTH-FACTORS STIMULATE TYROSINE DEPHOSPHORYLATION OF P75 AND ITS DISSOCIATION FROM THE SH2 DOMAIN OF GRB2

The growth factor receptor binding protein (Grb2) has a key role in in itiating the mitogen-activated protein kinase signaling cascade in maj or cell regulatory pathways. The binding of proteins to the SH2 domain of Grb2 has been reported to occur mainly after they are tyrosine-pho sphorylated following receptor activation. Using an in vitro binding a ssay, immunoprecipitation, and Far Western techniques, we report that in quiescent cells a 75-kDa protein binds directly to the SH2 domain o f Grb2. All of the tyrosine-phosphorylated p75 protein co-localizes wi th Grb2.Sos complex in the cytosolic fraction of the cell in vivo and undergoes tyrosine dephosphorylation when cells are treated with mitog enic ligands such as epidermal, platelet derived, and fibroblast growt h factors, endothelin-1, and bombesin but not tumor necrosis factor-al pha, interferon-alpha and -gamma, interleukein-6, and leukemic inhibit ory factor, which are either cell growth inhibitory or not significant ly mitogenic. The dephosphorylation of p75 and the ensuing dissociatio n from Grb2 is rapid, occurring within 30 s following mitogenic stimul ation by ligands such as epidermal growth factor, suggesting p75 to be an early component in the signal transduction pathways involving Grb2 .