2.A.55 The Metal Ion (Mn2+-iron) Transporter (Nramp) Family
Homologues of this family are found in various yeasts, plants, animals, archaea, and Gram-negative and Gram-positive bacteria termed ''natural resistance-associated'' macrophage protein (NRAMP) proteins because one of the animal homologues plays a role in resistance to intracellular bacterial pathogens such as Salmonella enterica serovar Typhimurium, Leishmania donovani and Mycobacterium bovis. The natural history of SLC11 genes in vertebrates has been discussed by Neves et al. (2011). Proposed to be a distant member of the DMT Superfamily (TC# 2.A.7), several human pathologies may result from defects in Nramp-dependent Fe2+ or Mn2+ transport, including iron overload, neurodegenerative diseases and innate susceptibility to infectious diseases (Cellier 2012).
Humans and rodents possess two distinct NRAMPs. The broad specificity NRAMP2 (DMT1), which transports a range of divalent metal cations, transports Fe2+ and H+ with a 1:1 stoichiometry and apparent affinities of 6 μm and about 1 μm, respectively. Variable H+:Fe2+ stoichiometry has also been reported. The order of substrate preference for NRAMP2 is Fe2+> Zn2+> Mn2+> Co2+> Ca2+> Cu2+> Ni2+> Pb2+. Many of these ions can inhibit iron absorption. Mutation of Nramp2 in rodents leads to defective endosomal iron export within the ferritin cycle, impaired intestinal iron absorption and microcytic anemia. Symptoms of Mn2+ deficiency are also seen. It is found in apical membranes of intestinal epithelial cells but also in late endosomes and lysosomes.
In contrast to the widely expressed NRAMP2, NRAMP1 is expressed primarily in macrophages and monocytes and appears to have a preference for Mn2+ rather than Fe2+. NRAMP1 (TC# 2.A.55.2.3) has been reported to function by metal:H+ antiport (Techau et al., 2007). It is hypothesized that a deficiency for Mn2+ or some other metal prevents the generation of reactive oxygenic and nitrogenic compounds that are used by macrophage to combat pathogens. This hypothesis is supported by studies on the bacterial NRAMP homologues which exhibit extremely high selectivity for Mn2+ over Fe2+, Zn2+ and other divalent cations. Regulation of these transporters in bacteria can occur through Fur, OxyR, and most commonly a DtxR homolog, MntR.
The Smf1 protein of Saccharomyces cerevisiae appears to catalyze high-affinity (Km = 0.3 μm) Mn2+ uptake while the closely related Smf2 protein may catalyze low affinity (Km = 60 μm) Mn2+ uptake in the same organism. Both proteins also mediate H+-dependent Fe2+ uptake. These proteins are of 575 and 549 amino acyl residues in length and are predicted to have 8-12 transmembrane α-helical spanners. The E. coli homologue of 412 aas exhibits 11 putative and confirmed TMSs with the N-terminus in and the C-terminus out. The yeast proteins may be localized to the vacuole and/or the plasma membrane of the yeast cell. Indirect and some direct experiments suggest that they may be able to transport several heavy metals including Mn2+, Cu2+, Cd2+ and Co2+. A third yeast protein, Smf3p, appears to be exclusively intracellular, possibly in the Golgi. Nramp2 (Slc11A2) of Homo sapiens (TC #2.A.55.2.1) has a 12 TMS topology with intracellular N- and C-termini. Two-fold structural symmetry in the arrangement of membrane helices for TM1-5 and TM6-10 (conserved Slc2 hydrophobic core) is suggested (Czachorowski et al., 2009).
The Nramp family of divalent metal transporters enables manganese import in bacteria and dietary iron uptake in mammals. Bozzi et al. 2016 determined the crystal structure of the Deinococcus radiodurans Nramp homolog (DraNramp) in an inward-facing apo state, including the complete transmembrane (TM) segment 1a (absent from a previous Nramp structure). Cysteine accessibility scanning results Allowed identification of the metal-permeation pathway in the alternate outward-open conformation. Two natural anemia-causing glycine-to-arginine mutations impaired transition metal transport in both human Nramp2 and DraNramp. The TM4 G153R mutation perturbs the closing of the outward metal-permeation pathway and alters the selectivity of the conserved metal-binding site. In contrast, the TM1a G45R mutation prevents conformational change by sterically blocking the essential movement of that helix, thus locking the transporter in an inward-facing state (Bozzi et al. 2016).
The generalized transport reaction catalyzed by Nramp family proteins is:
Me2+ (out) H+ (out) ⇌ Me2+ (in) H+ (in)