Expression and Mutagenesis of ZntA, a Zinc-Transporting P-Type ATPase from Escherichia coli

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Abstract
Cation-transporting P-type ATPases comprise a major membrane protein family, the members of which are found in eukaryotes, eubacteria, and archaea. A phylogenetically old branch of the P-type ATPase family is involved in the transport of heavy-metal ions such as copper, silver, cadmium, and zinc. In humans, two homologous P-type ATPases transport copper. Mutations in the human proteins cause disorders of copper metabolism known as Wilson and Menkes diseases. E. coli possesses two genes for heavy-metal translocating P-type ATPases. We have constructed an expression system for one of them, ZntA, which encodes a 732 amino acid residue protein capable of transporting Zn2+. A vanadate-sensitive, Zn2+-dependent ATPase activity is present in the membrane fraction of our expression strain. In addition to Zn2+, the heavy-metal ions Cd2+, Pb2+, and Ag+ activate the ATPase. Incubation of membranes from the expression strain with [γ-33P]ATP in the presence of Zn2+, Cd2+, or Pb2+ brings about phosphorylation of two membrane proteins with molecular masses of approximately 90 and 190 kDa, most likely representing the ZntA monomer and dimer, respectively. Although Cu2+ can stimulate phosphorylation by [γ-33P]ATP, it does not activate the ATPase. Cu2+ also prevents the Zn2+ activation of the ATPase when present in 2-fold excess over Zn2+. Ag+ and Cu+ appear not to promote phosphorylation of the enzyme. To study the effects of Wilson disease mutations, we have constructed two site-directed mutants of ZntA, His475Gln and Glu470Ala, the human counterparts of which cause Wilson disease. Both mutants show a reduced metal ion stimulated ATPase activity (about 30−40% of the wild-type activity) and are phosphorylated much less efficiently by [γ-33P]ATP than the wild type. In comparison to the wild type, the Glu470Ala mutant is phosphorylated more strongly by [33P]Pi, whereas the His475Gln mutant is phosphorylated more weakly. These results suggest that the mutation His475Gln affects the reaction with ATP and Pi and stabilizes the enzyme in a dephosphorylated state. The Glu470Ala mutant seems to favor the E2 state. We conclude that His475 and Glu470 play important roles in the transport cycles of both the Wilson disease ATPase and ZntA.