The cation diffusion facilitator protein MamM's cytoplasmic domain exhibits metal-type dependent binding modes and discriminates against Mn2+

Cation diffusion facilitator (CDF) proteins are a conserved family of divalent transition metal cation transporters. CDF proteins are usually composed of two domains: the transmembrane domain (TMD), in which the metal cations are transported through, and a regulatory cytoplasmic C-terminal domain (CTD). Each CDF protein transports either one specific metal, or multiple metals, from the cytoplasm. Here, the model CDF protein MamM, from magnetotactic bacteria, was used to probe the role of the CTD in metal selectivity. Using a combination of biophysical and structural approaches, the binding of different metals to MamM CTD was characterized. Results reveal that different metals bind distinctively to MamM CTD in terms of; their binding sites, thermodynamics and binding-dependent conformation, both in crystal form and in solution. Furthermore, the results indicate that the CTD discriminates against Mn2+ and provides the first direct evidence that CDF CTD9s play a role in metal selectivity.