The present invention provides metal ion affinity peptides, fusion proteins comprising metal ion affinity peptides, and polynucleotides encoding the fusion proteins. A feature of the subject invention is that the metal ion affinity peptide has a formula selected from the group consisting of: formula 1: (His-X.sub.1-X.sub.2).sub.n1-(His-X.sub.3-X.sub.4-X.sub.5).sub.n2-(His-X.- sub.6).sub.n3, wherein each of X.sub.1 and X.sub.2 is independently an amino acid with an aliphatic or an amide side chain, each of X.sub.3, X.sub.4, X.sub.5 is independently an amino acid with a basic side chain (except His) or an acidic side chain, each X.sub.6 is an amino acid with an aliphatic or an amide side chain, n1 and n2 are each independently 1 3, and n3 is 1 5; formula 2: (His-Asn).sub.n, where n=3 to 10; and formula 3: (His-X.sub.1-X.sub.2).sub.n, wherein each of X.sub.1 and X.sub.2 is an amino acid having an acidic side chain, and n=3 to 10. The invention further provides recombinant vectors comprising subject polynucleotides, and host cells comprising the recombinant vectors. The invention further provides methods and kits for purifying a fusion protein comprising a metal ion affinity peptide.