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The divalent cation binding properties of human prothymosin a, an abundant nuclear protein involved in cell proliferation, were evaluated. By using prothymosin a retardation on a weak cation chelating resin charged with various divalent cations, specific binding of Zn 21 ions by prothymosin a was observed. This finding was further confirmed by the equilibrium dialysis analysis which demonstrated that, within the micromolar range of Zn 21 concentrations, prothymosin a could bind up to three zinc ions in the presence of 100 mm NaCl and up to 13 zinc ions in the absence of NaCl. Equilibrium dialysis analysis also revealed that prothymosin a could bind Ca 21 , although the parameters of Ca 21 binding by prothymosin a were less pronounced than those of Zn 21 binding in terms of the number of metal ions bound, the K D values, and the resistance of the bound metal ions to 100 mm NaCl. The effects of Zn 21 and Ca 21 on the interaction of prothymosin a with its putative partners, Rev of HIV type 1 and histone H1, were examined. We demonstrated that Rev binds prothymosin a, and that prothymosin a binding to Rev but not to histone H1 was significantly enhanced in the presence of zinc and calcium ions. Our data suggest that the modes of prothymosin a interaction with Rev and histone H1 are distinct and that the observed zinc and calcium-binding properties of prothymosin a might be functionally relevant.

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