The mechanism of ATP hydrolysis of a shortened variant of the heavy metal-translocating P-type ATPase CopB ofSulfolobus solfataricus was studied. The catalytic fragment, named CopB-B, comprises the nucleotide binding and phosphorylation domains. We demonstrated stoichiometric high-affinity binding of one nucleotide to the protein (Kdiss 1–20lm). Mg is not necessary for nucleotide association but is essential for the phospha-tase activity.