Conversion of a phosphoseryl/threonyl phosphatase into a phosphotyrosyl phosphatase

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Abstract
By use of the autophosphorylated epidermal-growth-factor receptor and the synthetic peptide RRLIE-DAEY(P)AARG, representing an autophosphorylation site of the transforming protein of Rous-sarcoma virus, it is demonstrated that the phosphotyrosyl phosphatase activity of the polycation-stimulated phosphatases is substantially increased by an enzyme-directed effect of ATP or PP1. Concomitant with this increase in phosphotyrosyl phosphatase activity, the phosphorylase phosphatase activity is decreased, thus dramatically changing the substrate specificity of these enzymes. The dephosphorylation of four different phosphotyrosyl sites of the epidermal-growth-factor receptor is neither consecutive nor at random, but a preferred dephosphorylation of the P1 site over the P3 > P2 > P4 sites is observed. This phosphatase activity represents a substantial fraction of the total phosphotyrosyl phosphatase activity in the post-mitochondrial supernatant of Xenopus laevis oocytes.