Divalent metal cation binding properties of human prothymosin alpha

The divalent cation binding properties of human prothymosin alpha, an abund ant nuclear protein involved in cell proliferation, were evaluated. By usin g prothymosin alpha retardation on a weak cation chelating resin charged wi th various divalent cations, specific binding of Zn2+ ions by prothymosin a lpha was observed. This finding was further confirmed by the equilibrium di alysis analysis which demonstrated that, within the micromolar range of Zn2 + concentrations, prothymosin alpha 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. Equ ilibrium dialysis analysis also revealed that prothymosin alpha could bind Ca2+, although the parameters of Ca2+ binding by prothymosin alpha were les s pronounced than those of Zn2+ binding in terms of the number of metal ion s bound, the K-D values, and the resistance of the bound metal ions to 100 mm NaCl. The effects of Zn2+ and Ca2+ on the interaction of prothymosin alp ha with its putative partners, Rev of HIV type 1 and histone H1, were exami ned. We demonstrated that Rev binds prothymosin alpha, and that prothymosin alpha binding to Rev but not to histone H1 was significantly enhanced in t he presence of zinc and calcium ions. Our data suggest that the modes of pr othymosin alpha interaction with Rev and histone H1 are distinct and that t he observed zinc and calcium-binding properties of prothymosin alpha might be functionally relevant.