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 %u03BCM and about 1 %u03BCM, 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 (K_m = 0.3 %u03BCM) Mn² uptake while the closely related Smf2 protein may catalyze low affinity (K_m = 60 %u03BCM) 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 generalized transport reaction catalyzed by Nramp family proteins is:

Me² (out) H (out) %u2192 Me² (in) H (in).