Structural and functional basis of proton-independent transition metal import by a canonical bacterial Nramp transporter

Natural resistance-associated macrophage proteins (Nramps) are divalent transition metal transporters found in most organisms, typically coupling metal uptake to proton co-transport. How this coupling evolved, however, remains unclear. We present structural, functional, and evolutionary analyses of a clade B Nramp from the gut bacterium Bacteroides fragilis (BfraNramp). Phylogenetic reconstruction positions clade B as the most basal group of canonical Nramps, retaining conserved metal-binding motifs while lacking most residues that form the canonical proton pathway. We show that BfraNramp efficiently transports Mn2+ and Cd2+ with high apparent affinity but without proton co-transport or dependence on membrane potential or pH. Structures of metal-free and Mn2+-bound BfraNramp reveal an inward-open conformation and a distinct metal coordination geometry involving a conserved glutamate on transmembrane helix 3. Together, these results identify clade B Nramps as proton-independent transition metal uniporters and suggest that proton coupling emerged later in Nramp evolution, following establishment of the metal-binding site.