Analysis of Metal-Binding Features of the Wild Type and Two Domain-Truncated Mutant Variants of Littorina littorea Metallothionein Reveals Its Cd-Specific Character

Abstract

After the resolution of the 3D structure of the Cd₉-aggregate of the <i>Littorina littorea</i> metallothionein (MT), we report here a detailed analysis of the metal binding capabilities of the wild type MT, LlwtMT, and of two truncated mutants lacking either the N-terminal domain, Lltr2MT, or both the N-terminal domain, plus four extra flanking residues (SSVF), Lltr1MT. The recombinant synthesis and in vitro studies of these three proteins revealed that LlwtMT forms unique M₉-LlwtMT complexes with Zn(II) and Cd(II), while yielding a complex mixture of heteronuclear Zn,Cu-LlwtMT species with Cu(I). As expected, the truncated mutants gave rise to unique M₆-LltrMT complexes and Zn,Cu-LltrMT mixtures of lower stoichiometry with respect to LlwtMT, with the SSVF fragment having an influence on their metal binding performance. Our results also revealed a major specificity, and therefore a better metal-coordinating performance of the three proteins for Cd(II) than for Zn(II), although the analysis of the Zn(II)/Cd(II) displacement reaction clearly demonstrates a lack of any type of cooperativity in Cd(II) binding. Contrarily, the analysis of their Cu(I) binding abilities revealed that every LlMT domain is prone to build Cu₄-aggregates, the whole MT working by modules analogously to, as previously described, certain fungal MTs, like those of <i>C. neoformans</i> and <i>T. mesenterica</i>. It is concluded that the <i>Littorina littorea</i> MT is a Cd-specific protein that (beyond its extended binding capacity through an additional Cd-binding domain) confers to <i>Littorina littorea</i> a particular adaptive advantage in its changeable marine habitat.