NONCATALYTIC ACTIVATION OF PHOSPHOLIPASE C-GAMMA(1) IN-VITRO BY EPIDERMAL GROWTH-FACTOR RECEPTOR

To investigate the possible functional role of epidermal growth factor (EGF) receptor-phospholipase C-gamma1 (PLC-gamma1) complexes, we have measured PLC-gamma1 activity in vitro in the absence or presence of purified EGF receptor. Immunoprecipitates of PLC-gamma1 from control A-431 cells were incubated without or with purified EGF receptor in the absence or presence of ATP. Under these conditions the EGF receptor increased non-tyrosine-phosphorylated PLC-gamma1 activity 3-4-fold in the absence or presence of ATP, but increased tyrosine-phosphorylated and activated PLC-gamma1 by only 20-50%. Both basal and autophosphorylated forms of the purified EGF receptor increased the activity of the non-tyrosine-phosphorylated PLC-gamma1, and stoichiometric levels of purified receptor were required to increase PLC activity. Other tyrosine kinases such as the platelet-derived growth factor receptor and erbB-2, but not the insulin receptor, also stimulated PLC-gamma1 activity. PLC-gamma1 activity could be activated with the kinase-negative EGF receptor, but a C-terminal truncated receptor was much less effective. Purified EGF receptor could also activate PLC-beta1, but with a much decreased potency compared with PLC-gamma1. Our results suggest that in vitro the EGF receptor can increase PLC-gamma1 activity independently of tyrosine phosphorylation.