The cadmium binding domains in the metallothionein isoform Cd7-MT10 from Mytilus galloprovincialis revealed by NMR spectroscopy

The metal–thiolate connectivity of recombinant Cd₇-MT10 metallothionein from the sea mussel Mytilus galloprovincialis has been investigated for the first time by means of multinuclear, multidimensional NMR spectroscopy. The internal backbone dynamics of the protein have been assessed by the analysis of ¹⁵N T ₁ and T ₂ relaxation times and steady state {¹H}–¹⁵N heteronuclear NOEs. The ¹¹³Cd NMR spectrum of mussel MT10 shows unique features, with a remarkably wide dispersion (210 ppm) of ¹¹³Cd NMR signals. The complete assignment of cysteine Hα and Hβ proton resonances and the analysis of 2D ¹¹³Cd–¹¹³Cd COSY and ¹H–¹¹³Cd HMQC type spectra allowed us to identify a four metal–thiolate cluster (α-domain) and a three metal–thiolate cluster (β-domain), located at the N-terminal and the C-terminal, respectively. With respect to vertebrate MTs, the mussel MT10 displays an inversion of the α and β domains inside the chain, similar to what observed in the echinoderm MT-A. Moreover, unlike the MTs characterized so far, the α-domain of mussel Cd₇-MT10 is of the form M₄S₁₂ instead of M₄S₁₁, and has a novel topology. The β-domain has a metal–thiolate binding pattern similar to other vertebrate MTs, but it is conformationally more rigid. This feature is quite unusual for MTs, in which the β-domain displays a more disordered conformation than the α-domain. It is concluded that in mussel Cd₇-MT10, the spacing of cysteine residues and the plasticity of the protein backbone (due to the high number of glycine residues) increase the adaptability of the protein backbone towards enfolding around the metal–thiolate clusters, resulting in minimal alterations of the ideal tetrahedral geometry around the metal centres.