Structures in multiple conformations reveal distinct transition metal and proton pathways in an Nramp transporter

Abstract

Nramp family transporters--expressed in organisms from bacteria to humans--enable uptake of essential divalent transition metals via an alternating-access mechanism that includes proton co-transport. We present high-resolution structures of Deinococcus radiodurans (Dra)Nramp at complementary stages of its transport cycle to provide a thorough description of the Nramp transport cycle by identifying the key intramolecular rearrangements and changes to the metal coordination sphere. Strikingly, while metal transport requires cycling from outward-to inward-open states, efficient proton transport still occurs in outward-locked (but not inward-locked) DraNramp. We propose a model in which metal and proton enter the transporter via the same external pathway to the binding site, but follow separate routes to the cytoplasm, thus resolving the electrostatic dilemma of using a cation co-substrate to drive a cation primary substrate. Our results illustrate the flexibility of the LeuT fold to support a broad range of co-substrate coupling and conformational change mechanisms.