Slc11a1 (formerly Nramp1) is a proton/divalent cation transporter that regulates cation homeostasis in macrophages. Slc11a2 mediates divalent cation uptake via the gut and delivery into cells. The mode of action of the two transporters remains controversial. Heterologous expression in frog oocytes shows Slc11a2 is a symporter, whereas Slc11a1 is an antiporter fluxing divalent cations against the proton gradient. This explains why Slc11a2, but not Slc11a1, can complement EGTA sensitivity in smf1⌬/smf2⌬/smf3⌬ yeast. However, some studies of transport in mammalian cells suggest Slc11a1 is a symporter. We now demonstrate that Slc11a1, but not Slc11a2, complements a divalent cation stress phenotype in bsd2⌬/rer1⌬ yeast. This is the first description of a yeast complementation assay for Slc11a1 function. Given the prior demonstration in frog oocytes that Slc11a1 acts as an antiporter, the most plausible interpretation of the data is that Slc11a1 is rescuing bsd2⌬/rer1⌬ yeast by exporting divalent cations. Chimaeras define the N terminus, and a segment of the protein core preceding transmembrane domain 9 through transmembrane domain 12, as important in rescuing the divalent cation stress phenotype. EGTA sensitivity and divalent cation stress phenotypes in yeast expressing Slc11a orthologues show that symport activity is ancestral. Molecular changes that mediate rescue of the divalent cation stress phenotype post-date frogs and co-evolved with Slc11a1 orthologues that regulate divalent cation homeostasis in macrophages and resistance to infection in chickens and mammals.Slc11a1 (formerly Ity/Lsh/Bcg/Nramp1) is a proton/divalent cation transporter with 12 putative transmembrane domains (TMD) 4 (1) that localizes to late endosomes and lysosomes of macrophages (2, 3) and dendritic cells (4), and tertiary granules of neutrophils (5). It is recruited to phagosome membranes in macrophages infected with Mycobacterium (2, 3) or Leishmania (3), and regulates cellular divalent cation homeostasis (6, 7). Polymorphisms at murine Slc11a1 and human SLC11A1 contribute to susceptibility to numerous infectious and autoimmune diseases (8, 9). Slc11a2 (also Nramp2/DMT1/DCT1) shares 64% amino acid identity with Slc11a1 (10) and regulates divalent cation uptake via the gut and delivery of iron into multiple cell types (11,12). Different isoforms can be distinguished by different C-terminal amino acid sequences, and by the presence or absence of an iron response element located in the 3Ј-untranslated region of the mRNA (13). The IRE-containing isoform (Slc11a2-IRE) is expressed predominantly in epithelial cells and localizes to late endosome/lysosomes, whereas the non-IRE-containing isoform (Slc11a2-nonIRE) is expressed in blood cell lineages and localizes to early endosomes (13). Both isoforms localize to their respective endosomal compartments and to apical membranes when expressed in polarized MadinDarby canine kidney cells (13). Mutation at Slc11a2 in mice and rats is associated with microcytic anemia (14, 15).Fe 2ϩ , Zn 2ϩ , Mn 2ϩ , Co ...

A Xenopus oocyte heterologous expression system was used to characterise iron transport properties of two members of the solute carrier 11 (slc11) protein family isolated from rainbow trout gills. One cDNA clone differed from the trout Slc11a containing an additional 52 bp in the exon between transmembrane domains (TM) 10 and 11. The 52 bp contained a stop codon, resulting in a novel isoform lacking the last two TM (termed slc11c). Slc11c and another isoform slc11b, import Fe 2+ at external pHs 6 to 7.4. Trout slc11b Fe 2+ import was more sensitive to inhibition by divalent metals. The novel vertebrate slc11c isoform functions without TM11 and 12.