A site of tyrosine phosphorylation in the C terminus of the epidermal growth factor receptor is required to activate phospholipase C.

Cells expressing mutant epidermal growth factor (EGF) receptors have been used to study mechanisms through which EGF increases phospholipase C (PLC) activity. C-terminal truncation mutant EGF receptors are markedly impaired in their ability to increase inositol phosphate formation compared with wild-type EGF receptors. Mutation of the single tyrosine self-phosphorylation site at residue 992 to phenylalanine in an EGF receptor truncated at residue 1000 abolished the ability of EGF to increase inositol phosphate formation. C-terminal deletion mutant receptors that are impaired in their ability to increase inositol phosphate formation effectively phosphorylate PLC-gamma at the same tyrosine residues as do wild-type EGF receptors. EGF enhances PLC-gamma association with wild-type EGF receptors but not with mutant receptors lacking sites of tyrosine phosphorylation. These results indicate that formation of a complex between self-phosphorylated EGF receptors and PLC-gamma is necessary for enzyme activation in vivo. We propose that both binding of PLC-gamma to activated EGF receptors and tyrosine phosphorylation of the enzyme are necessary to elicit biological responses. Kinase-active EGF receptors lacking sites of tyrosine phosphorylation are unable to signal increased inositol phosphate formation and increases in cytosolic Ca2+ concentration.