Structural and functional properties of a magnesium transporter of the SLC11/NRAMP family.

Karthik Ramanadane,Raimund Dutzler,Monique S Straub,Cristina Manatschal

doi:10.7554/elife.74589

Abstract

Members of the ubiquitous SLC11/NRAMP family catalyze the uptake of divalent transition metal ions into cells. They have evolved to efficiently select these trace elements from a large pool of Ca2+ and Mg2+, which are both orders of magnitude more abundant, and to concentrate them in the cytoplasm aided by the cotransport of H+ serving as energy source. In the present study, we have characterized a member of a distant clade of the family found in prokaryotes, termed NRMTs, that were proposed to function as transporters of Mg2+. The protein transports Mg2+ and Mn2+ but not Ca2+ by a mechanism that is not coupled to H+. Structures determined by cryo-EM and X-ray crystallography revealed a generally similar protein architecture compared to classical NRAMPs, with a restructured ion binding site whose increased volume provides suitable interactions with ions that likely have retained much of their hydration shell.

Highlights

Divalent cations constitute important factors in numerous biological processes and they are essential nutrients that need to be imported into cells in the required amounts
The signature is fully conserved in Fe2+ transporters from animals, bacterial Mn2+ transporters and most transporters of plant origin, which all exhibit a strong selectivity against alkaline earth metal ions but poor discrimination between transition metal ions
Due to the sensitivity of the assay and the fact that binding site mutants of EcoDMT retained their ability to transport Mn2+, we initially investigated transport properties of EleNRMT for this divalent transition metal ion and found robust concentration-dependent activity that saturates with a Km of around 120 μM for either WT or the thermostabilized mutant 195 (Figure 3A, B, D, E)

Summary

Introduction

Divalent cations constitute important factors in numerous biological processes and they are essential nutrients that need to be imported into cells in the required amounts. The transition metal ion Fe2+ plays a central role in oxygen transport and, together with Mn2+, it is a cofactor of enzymes catalyzing redox reactions Due to their low abundance in the extracellular environment, the uptake of transition metals has to proceed with high selectivity to prevent competition with several orders of magnitude more abundant alkaline earth metals, which would prohibit their efficient accumulation. This challenge is overcome by specific transmembrane transport proteins, which catalyze the accumulation of transition metal ions inside cells. Besides the transport of both transition metals, plant NRAMPs are involved in detoxification processes by transporting noxious metal ions such as Cd2+ (Huang et al, 2020a)

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Conclusion