Insulin-receptor phosphotyrosyl-protein phosphatases

Abstract

Calmodulin-dependent protein phosphatase has been proposed to be an important phosphotyrosyl-protein phosphatase. The ability of the enzyme to attack autophosphorylated insulin receptor was examined and compared with the known ability of the enzyme to act on autophosphorylated epidermal-growth-factor (EGF) receptor. Purified calmodulin-dependent protein phosphatase was shown to catalyse the complete dephosphorylation of phosphotyrosyl-(insulin receptor). When compared at similar concentrations, 32P-labelled EGF receptor was dephosphorylated at greater than 3 times the rate of 32P-labelled insulin receptor; both dephosphorylations exhibited similar dependence on metal ions and calmodulin. Native phosphotyrosyl-protein phosphatases in cell extracts were also characterized. With rat liver, heart or brain, most (75%) of the native phosphatase activity against both 32P-labelled insulin and EGF receptors was recovered in the particulate fraction of the cell, with only 25% in the soluble fraction. This subcellular distribution contrasts with results of previous studies using artificial substrates, which found most of the phosphotyrosyl-protein phosphatase activity in the soluble fraction of the cell. Properties of particulate and soluble phosphatase activity against 32P-labelled insulin and EGF receptors are reported. The contribution of calmodulin-dependent protein phosphatase activity to phosphotyrosyl-protein phosphatase activity in cell fractions was determined by utilizing the unique metal-ion dependence of calmodulin-dependent protein phosphatase. Whereas Ni2+ (1 mM) markedly activated the calmodulin-dependent protein phosphatase, it was found to inhibit potently both particulate and soluble phosphotyrosyl-protein phosphatase activity. In fractions from rat liver, brain and heart, total phosphotyrosyl-protein phosphatase activity against both 32P-labelled receptors was inhibited by 99.5 +/- 6% (mean +/- S.E.M., 30 observations) by Ni2+. Results of Ni2+ inhibition studies were confirmed by other methods. It is concluded that in cell extracts phosphotyrosyl-protein phosphatases other than calmodulin-dependent protein phosphatase are the major phosphotyrosyl-(insulin receptor) and -(EGF receptor) phosphatases.