Abstract
The amino-acid sequence of a metallothionein is reported. Metallothionein is a widely distributed, extremely cysteine-rich, low-molecular-weight protein containing large amounts of cadmium and/or zinc. Metallothionein-1B is one of the two prinicipal variants occurring in equine kidney cortex. The single-chain protein contains 61 amino acids and has the composition Cys20 Ser8Lys7Arg1Ala7Gly5Val3Asp2Asn1-Glu1Gln2Pro2Thr1Met1(Cd + Zn)7. Its amino-terminal residue is N-acetylmethionine. The sequence shows distinct clustering of the twenty cysteinyl residues into seven groups separated by stretches of at least three other residues. Within these groups the cysteines occur seven times in alternating Cys-X-Cys sequences and three times each in Cys-Cys and Cys-X-X-Cys sequences, where X is an amino acid other than cysteine. Another unique feature is the close association of serine and the basic amino acids with cysteine, as manifested by the occurrence of seven Ser-Cys, four Cys-Lys, one Cys-Arg, and three Lys-Cys sequences. These findings are in agreement with the previous suggestion that metallothionein has structurally defined metal-binding sites, most of which contain three cysteinyl residues as the principal metal-binding ligands. The charge difference between the metal-free and the metal-containing protein is consistent with the formation of negatively charged trimercaptide complexes with cadmium and/or zinc ions. The possible additional involvement of serine and the basic amino acids in metal binding is discussed.