Independence of the domains of metallothionein in metal binding.

Mammalian metallothionein is a low molecular weight protein with two metal-binding domains. To determine if metal binding in one domain affects binding in the other, we prepared peptides corresponding to the regions that enfold the two metal-thiolate clusters. Metal reconstitution studies of these peptides revealed stoichiometries of metal binding similar to those observed within the intact molecule. Thus, the alpha domain coordinates 4 Cd(II), 6 Cu(I), or 6 Ag(I) ions regardless of whether the domain is part of the total protein or is studied as a separate peptide. Likewise, the beta domain binds 3 Cd(II), 6 Cu(I), or 6 Ag(I) ions in both the intact protein and as a separate peptide. If cluster B in intact metallothionein is preformed with Cu(I) or Ag(I), cluster A saturates with either 4 mol eq of Cd(II) or 6 mol eq of Ag(I). Similarly, preformation of the A cluster with Cd(II) does not affect the binding of 6 Cu(I) ions in the B cluster. Therefore, the metal-dependent folding of the protein to create one cluster occurs independent of constraints or influences from the other domain. Formation of the protein with a tetrahedrally coordinated metal in one cluster and a trigonally coordinated metal in the other center is possible.