Structural and functional diversity of copper-metallothioneins from the American lobster Homarus americanus.

The role of copper metallothionein (CuMT) in copper metabolism and metalloenzyme activation is poorly understood. We have chosen marine crustaceans, in which a direct correlation exists between levels of Cu(I)MT and Cu(I)-hemocyanin during the molt cycle (Engel and Brouwer, Biol. Bull. 173, 239-251, 1987) as unique model systems to study the involvement of MTs in metalloprotein activation and degradation. We have isolated three low-molecular weight, cysteine-rich copper proteins from the American lobster Homarus americanus, which we designate as CuMT-1, CuMT-2, and CuMT-3, respectively. As a first attempt to fully characterize these proteins, we have determined the sequence of the first 56 amino acids of CuMT-1. The results show this protein to belong to the class I MTs, i.e., related in primary structure to equine renal MT. CuMT-1 cannot transfer its copper to copper-depleted apohemocyanin. CuMT-2 belongs to the same class of MTs as CuMT-1, but CuMT-3 does not. The latter can reactivate lobster hemocyanin containing reduced amounts of Cu(I). Spectroscopic studies show that Cu(I) transfer from CuMT-3 to apohemocyanin initially results in the formation of distorted binuclear-copper sites, which subsequently slowly return to their native stereochemical configuration. Finally, we present evidence that shows that the class I MTs in marine crustacea are involved in the sequestration of elevated levels of heavy-metal ions. These observations strongly suggest that the different forms of MT have different biological functions.