High-resolution structures with bound Mn²⁺ and Cd²⁺ map the metal import pathway in an Nramp transporter

Transporters of the Nramp (Natural resistance-associated macrophage protein) family import divalent transition metal ions into cells of most organisms. By supporting metal homeostasis, Nramps prevent diseases and disorders related to metal insufficiency or overload. Previous studies revealed that Nramps take on a LeuT fold and identified the metal-binding site. We present high-resolution structures of Deinococcus radiodurans (Dra)Nramp in three stable conformations of the transport cycle revealing that global conformational changes are supported by distinct coordination geometries of its physiological substrate, Mn²⁺, across conformations, and by conserved networks of polar residues lining the inner and outer gates. In addition, a high-resolution Cd²⁺-bound structure highlights differences in how Cd²⁺ and Mn²⁺ are coordinated by DraNramp. Complementary metal binding studies using isothermal titration calorimetry with a series of mutated DraNramp proteins indicate that the thermodynamic landscape for binding and transporting physiological metals like Mn²⁺ is different and more robust to perturbation than for transporting the toxic Cd²⁺ metal. Overall, the affinity measurements and high-resolution structural information on metal substrate binding provide a foundation for understanding the substrate selectivity of essential metal ion transporters like Nramps.