title: Conserved methionine dictates substrate preference in Nramp-family divalent metal transporters.
abstract: Natural resistance-associated macrophage protein (Nramp) family transporters catalyze uptake of essential divalent transition metals like iron and manganese. To discriminate against abundant competitors, the Nramp metal-binding site should favor softer transition metals, which interact either covalently or ionically with coordinating molecules, over hard calcium and magnesium, which interact mainly ionically. The metal-binding site contains an unusual, but conserved, methionine, and its sulfur coordinates transition metal substrates, suggesting a vital role in their transport. Using a bacterial Nramp model system, we show that, surprisingly, this conserved methionine is dispensable for transport of the physiological manganese substrate and similar divalents iron and cobalt, with several small amino acid replacements still enabling robust uptake. Moreover, the methionine sulfur's presence makes the toxic metal cadmium a preferred substrate. However, a methionine-to-alanine substitution enables transport of calcium and magnesium. Thus, the putative evolutionary pressure to maintain the Nramp metal-binding methionine likely exists because it-more effectively than any other amino acid-increases selectivity for low-abundance transition metal transport in the presence of high-abundance divalents like calcium and magnesium.
authors: Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138;
authors: Aaron T
authors: AT
authors: Bozzi
authors: Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138;
authors: Lukas B
authors: LB
authors: Bane
authors: Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138;
authors: Wilhelm A
authors: WA
authors: Weihofen
authors: Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138;
authors: Anne L
authors: AL
authors: McCabe
authors: Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801;
authors: Abhishek
authors: A
authors: Singharoy
authors: Laboratoire International Associé Centre National de la Recherche Scientifique et University of Illinois at Urbana-Champaign, Unité Mixte de Recherche CNRS n°7565, Université de Lorraine, 54506 Vandoeuvre-lès-Nancy cedex, France; Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
authors: Christophe J
authors: CJ
authors: Chipot
authors: Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801; Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
authors: Klaus
authors: K
authors: Schulten
authors: Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138; gaudet@mcb.harvard.edu.
authors: Rachelle
authors: 0000-0002-9177-054X
authors: R
authors: Gaudet
mesh_terms: D000595:Amino Acid Sequence / Q000235:genetics; D001692:Biological Transport / Q000235:genetics; D002118:Calcium / Q000737:chemistry; D027682:Cation Transport Proteins / Q000737:chemistry* / Q000235:genetics; D002413:Cations, Divalent / Q000737:chemistry; D003035:Cobalt / Q000737:chemistry; D034301:Deinococcus / Q000737:chemistry; D017136:Ion Transport / Q000235:genetics; D007501:Iron / Q000737:chemistry*; D008345:Manganese / Q000737:chemistry*; D008715:Methionine / Q000737:chemistry* / Q000235:genetics; D013379:Substrate Specificity; D000099173:Natural Resistance-associated Macrophage Protein 1
chemical_list: D002118:Calcium; D027682:Cation Transport Proteins; D002413:Cations, Divalent; D003035:Cobalt; D007501:Iron; D008345:Manganese; D008715:Methionine; D000099173:Natural Resistance-associated Macrophage Protein 1
keywords: MntH; divalent metal transporter DMT1; hard-soft acid-base theory; ion selectivity filters; transition metals