Structural implications for heavy metal-induced reversible assembly and aggregation of a protein: The case of Pyrococcus horikoshii CutA

CutA is a small protein that appears to be involved in the mechanism of divalent metal cation tolerance in microorganisms. Here we report the crystal structure of Pyrococcus horikoshii CutA (PhoCutA), with and without Cu ²⁺, and its metal-binding properties. Crystallographic analyses revealed that PhoCutA forms a stable trimeric structure with intertwined antiparallel β-strands. The crystal structure of the Cu²⁺- PhoCutA complex shows that the Cu²⁺ is located at a trimer-trimer interface and is recognized by the side chains of one Asp⁴⁸ from each trimer. In an in vitro experiment, PhoCutA bound to several heavy metals, most of which led to reversible aggregation of the protein; i.e. the aggregates could be completely solubilized by addition of ethylenediamine tetraacetic acid (EDTA) or dialysis against metal free buffer. Substitution of Asp⁴⁸ with Ala led to a decrease in the amount of aggregates, suggesting the significant contribution of Asp⁴⁸ to the reversible aggregation. To the best of our knowledge, this is the first report which provides the structural evidence for heavy metal-induced multimerization of a protein.