Multiple autophosphorylation sites of the epidermal growth factor receptor are essential for receptor kinase activity and internalization: Contrasting significance of tyrosine 992 in the native and truncated receptors

The role of epidermal growth factor (EGF) receptor autophosphorylation sites in the regulation of receptor functions has been studied using cells transfected with mutant EGF receptors. Simultaneous point mutation of 4 tyrosines (Y1068, Y1086, Y1148, Y1173) to phenylalanine, as well as removal of these sites by truncation of the carboxyl-terminal 123 amino acid residues, resulted in reduced receptor phosphorylation of an in vivo specific substrate phospholipase C-γ1 to less than 50% compared to the wild-type receptor. The internalization rate constant K_e was also significantly lower in these mutants (0.15/min) compared to cells transfected with wild-type receptor (0.27/min). Additional mutation of tyrosine 992 to phenylalanine in the truncated receptor mutant (Dc- 123F) further decreased the receptor internalization rate to a minimal level (k_e = 0.07-0.10/min), equivalent to the k_e measured for cells expressing kinase-negative receptor (A721). Moreover, tyrosine kinase activity of the Dc-123F receptor toward phospholipase C-γ1, compared to wild-type receptor, was reduced by 90%. Taken together, these results show that EGF receptor lacking five autophosphorylation sites functions similar to a kinase-negative receptor. Mutation of tyrosine residue Y992 alone in the context of full length EGF receptor, however, did not affect receptor internalization or kinase activity toward phospholipase C-γ1. These data indicate that tyrosine 992 is critical for substrate phosphorylation and internalization only in the context of the truncated receptor, and that minor autophosphorylation sites, such as Y992, may act as compensatory regulatory sties in the absence of the major EGF receptor autophosphorylation sites.