Abstract
A431 cell membranes phosphorylate a synthetic peptide (Arg-Arg-Leu-Ile-Glu-Asp-Asn-Glu-Tyr-Thr-Ala-Arg-Gly) in which residues 2--12 correspond to the sequence of the reported site of tyrosine phosphorylation in pp60src. Epidermal growth factor stimulates the phosphorylation of this peptide 2-fold over basal levels in a dose-dependent fashion. Phosphorylation is linear for approximately 3 min at 30 degrees C and occurs on the tyrosine residue. Kinetic analysis of the phosphorylation reaction indicates that epidermal growth factor increases the average Vmax from 3.8 to 7.5 nmol/min per mg and slightly decreases the average Km from 0.53 mM to 0.28 mM. A number of other peptides analogous to this tridecapeptide are also phosphorylated by A431 membranes. The data suggest that peptides with sequences similar to the site of tyrosine phosphorylation in pp60src are preferred substrates for the kinase in these membranes. Thus, the epidermal growth factor-stimulated protein kinase has the potential to interact with and phosphorylate pp60src. However, the A431 membranes also phosphorylate a tyrosine-containing peptide of totally unrelated sequence, suggesting that the kinase possesses a broad specificity for peptide phosphorylation that may not reflect its specificity with protein substrates.