Effects of Zn2+ ions on protein phosphorylation in epithelial cell membranes

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Abstract
The regulation of protein phosphorylation by Zn2+ ions and by other divalent cations was studied in membrane vesicles from a normal mouse epithelial cell line, MMC‐E (Mus musculus castaneous). Four major phosphoacceptor polypeptides were found in these membranes. Micromolar concentrations of Zn2+ ions inhibited the phosphorylation of the epidermal growth factor (EGF) receptor and of threonine residues in a 47,000‐dalton polypeptide. In contrast, two polypeptides with molecular weights of 54,000 and 57,000 showed increased phosphorylation, mainly of serine residues, in the p. esence of Zn2+ ions. These results were not obtained using similar concentrations of other divalent cations and were apparently not due to an effect of Zn2+ ions on phosphoprotein phosphatases. Thus, the effects of Zn2+ ions on protein phosphorylation in membrane vesicles are complex and are not restricted to an inhibition of a single protein phosphatase or kinase.