Copper(I)-mediated protein-protein interactions result from suboptimal interaction surfaces

Overview & Synopsis

The homeostasis of metal ions in cells is the result of the contribution of several cellular pathways that involve transient, often weak, protein-protein interactions. Metal transfer typically implies the formation of adducts where the metal itself acts as a bridge between proteins, by coordinating residues of both interacting partners. Here we addressed the interaction between the human copper(I)-chaperone HAH1 and one metal-binding domain of one of its partners, namely the P-type copper-transporting ATPase ATP7A. The adduct was structurally characterized in solution, in the presence of copper(I), and through X-ray crystallography, upon replacing copper(I) with cadmium(II). Further insight was obtained through molecular modelling techniques and site-directed mutagenesis. It is found that the interaction involves a relatively small interface (less than 1000 Å2) with a low fraction of non-polar atoms. These observations provide a possible explanation for the low affinity of the two apo-proteins. It appears that electrostatics is important in selecting which domain of the ATPase is able to form detectable amounts of the metal-mediatd adduct with HAH1.