Initially detected as a persistent contaminant in immobilized metal affinity chromatography of recombinant proteins in Escherichia coli, a 196-amino acid protein was isolated, cloned, overexpressed, and characterized. It consists of two domains, of which the first (146 amino acids) shows some homology to the FK506-binding proteins. The second domain (50 amino acids) is extremely rich in potentially metal-binding amino acids, such as histidine, cysteine, and acidic amino acids. The protein binds Ni2+ and Zn2+ tightly with 1:1 stoichiometry, Cu2+ and Co2+ with lower affinity, and Mn2+, Fe2+, Fe3+, Mg2+, and Ca2+ hardly at all.