The metal binding site composition of the cation diffusion facilitator protein MamM cytoplasmic domain impacts its metal responsivity

Abstract

The cation diffusion facilitator (CDF) is a conserved family of divalent d-block metal cation transporters that extrude these cations selectively from the cytoplasm. CDF proteins are composed of two domains: the transmembrane domain, through which the cations are transported, and a regulatory cytoplasmic C-terminal domain (CTD). Metal binding to the CTD leads to its tighter conformation, and this sequentially promotes conformational change of the transmembrane domain which allows the actual transport of specific metal cations. It was recently shown that the magnetotactic bacterial CDF protein MamM CTD has a role in metal selectivity, as binding of different metal cations exhibits distinctive affinities and conformations. It is yet unclear whether the composition of the CTD binding sites can impact metal selectivity. Here we performed a mutational study of MamM CTD, where we exchanged the metal binding residues with different metal-binding amino acids. Using X-ray crystallography and Trp-fluorescence spectrometry, we studied the impact of the mutations on the CTD conformation in the presence of different metals. Our results reveal that the incorporation of such mutations alters the domain response to metals in vitro, as mutant forms of the CTD bind metals differently in terms of the composition of the binding sites and the CTD conformation. CoordinatesMamM CTD structures have been deposited in the Protein Data Bank under the following accession codes: 6H5V, 6H5M, 6H5U, 6H8G, 6HAO, 6H88, 6H87, 6H8A, 6H89, 6H8D, 6H5K, 6H9Q, 6H84, 6H83, 6HA2, 6H8I, 6H9T, 6H81, 6HAN, 6H85, 6H9P, 6HHS.