2.A.108 The Iron/Lead Transporter (ILT) Family
The ILT family includes two families, the iron-transporting OFeT family and the lead-transporting PbrT family (see below). All sequenced homologues of the ILT family have been multiply aligned, and conserved motifs, including fully conserved acidic residues in putative transmembrane segments (TMSs) 1 and 4, previously implicated in heavy metal binding, have been identified (Debut et al. 2006). Topological analyses confirmed the presence of 7 conserved TMSs in a 3 + 3 + 1 arrangement where the two 3 TMS elements are internally repeated. Phylogenetic analyses revealed the presence of several sequence divergent clusters of orthologous proteins that group roughly according to the phylogenes of the organisms of origin (Debut et al., 2006).
Yeast (Saccharomyces cerevisiae, Candida albicans and Schizosaccharomyces pombe) and other fungi possess high affinity (Km ≈0.1 µM) Fe2+ uptake systems. These systems depend on cell surface ferroxidases to convert extracellular Fe2+ to Fe3+ which can then be taken up either via a low affinity (30 µM) transporter of the FeT family (TC #9.A.9) or a high affinity OFeT family transporter described here. Two gene products are required for high affinity Fe2+ transport, Fet3p which is the oxidase, and Ftr1p which is the permease component.
Fet3p of S. cerevisiae is a multicopper oxidase (636 amino acyl residues) which spans the plasma membrane once (residues 561-584) and has two multicopper oxidase domains (residues 121-141 and 483-494), which possess the ferroxidase activity on the external surface of the plasma membrane. It is a member of the multicopper oxidase family and is therefore homologous to laccase (benzenediol:oxygen oxidoreductase or ligninolytic phenol oxidase), as well as L-ascorbate oxidase, ceruloplasmin and dihydrogeodin oxidase. Its copper binding domain is homologous to that of the PcoA copper binding protein of E. coli.
Ftr1p is a protein of 404 amino acyl residues which may span the membrane seven times (Debut et al., 2006). It exhibits homology with other yeast ORFs as well as algal, bacterial and archaeal ORFs. The bacterial and archaeal ORFs are highly divergent from the yeast proteins and may therefore serve dissimilar functions. Recently a bacterial iron transporter has been characterized from a marine magnetotactic α-proteobacterium (Dubbels et al., 2004), but errors in the sequence precluded inclusion of this protein in TCDB.
Simultaneous expression of Fet3p and Ftr1p in yeast is required for proper localization of either protein at the cell surface, suggesting that a complex of the two proteins is formed. Both proteins are coordinately regulated, being expressed at high levels when iron is absent and repressed when iron is replete.
A group translocation reaction in which Fe2+ is simultaneously oxidized and transported to Fe3+ has been suggested but not demonstrated. Alternatively, Fe2+ may be oxidized by Fet3p to Fe3+ which may be passed from the Fet3p active site directly to the binding site for Fe3+ in Ftr1. Still another possibility is that Fet3p functions only indirectly in transport by allowing membrane insertion, localization or stability of Ftr1p due to the formation of a complex between these two proteins. Regardless of these possibilities, it is not known if a channel or carrier mechanism operates, and the nature of the energy coupling process for transport is not established.
A dipartite iron uptake system, FetM (646 aas; 8 TMSs in a 1 7 arrangement)/FetP (a periplasmic protein that enhances iron uptake by FetM) has been characterized (Koch et al. 2011). FetP binds Cu++ and Mn++ at two different sites, 1.3 Å apart, in this homodimeric protein. The 3-d structure with two Cu++ bound to each of the two subunits revealed different geometries at the two sites. FetMP may be a iron permease with an iron scavenging function, and possibly also an iron reducing function (Koch et al. 2011).
The generalized transport reactions for the OFeT family are:
(1) Fe3+ (out) → Fe3+ (in)
(2) Fe2+ (out) + 1/4 O2 (out) → Fe3+ (in) + 1/2 H2O (out)
A single protein, PbrT, encoded within the lead resistance locus of Ralstonia metallidurans CH34, serves as the prototype for the PbrT family. This protein, when overexpressed, increases sensitivity to Pb2+. The protein exhibits a single N-terminal hydrophobic segment (a putative TMS), plus 6 additional putative TMSs in the C-terminal region (residues 420-650) of this 652 aas protein (Debut et al., 2006). An N-terminal region (residues 100-218) shows sequence similarity to the C-terminal cytochrome C6 domain of the diheme c-type cytochrome, FixP, of Azorhizobium caulinodans (30% identity). The C-terminal transmembrane domain (residues 223-619) shows sequence similarity to members of the oxidase-dependent Fe2+ transporter, OFeT, family (TC #9.A.10) including the Ftr1 iron transporter of Saccharomyces cerevisiae (TC #9.A.10.1.1) (30% identity). Thus, PbrT is related to the OFeT family, both structurally and functionally. An N-terminal domain (residues 100-218 in the R. metallidurans protein) shows similarity to the C-terminal cytochrome C6 domain in the diheme c-type cytochrome, FixP of Azorhizobium caulinodans (Debut et al., 2006). We designate the family including both the OFeT and PbrT families the Fe/Pb-Transporter (ILT) superfamily.
Some bacteria have Fe2+ transporters that resemble PbrT more than members of the OFeT family. Members of both the OFeT and PbrT families contain two REXXE motifs in their TMSs. In EfeU of E. coli (TC# 9.A.10.2.3) the first motif is required for Fe2+ transport (Grosse et al. 2006).
The generalized transport reactions catalyzed by members of the PbrT family are:
Pb2+ (out) → Pb2+ (in)
Fe2+ (out) → Fe2+ (in)
High-affinity oxidase-dependent plasma membrane Fe2+-Fe3+ uptake transporter, Ftr1 of 404 aas and 7 TMSs in a 3. + 3 + 1 arrangement. May be a "transceptor", combining transport and receptor functions (Diallinas 2017).
Fet3 and Ftr1 of Saccharomyces cerevisiae
Fet3 (aka Fet3p) (multicopper ferrooxidase)
Ftr1 (Fe3+ transporter)
Iron-repressible Fe2+ uptake porter, FTR1 (Ziegler et al. 2011)
Ftr1 of Candida albicans
High-affinity oxidase-dependent vacuolar Fe2+-Fe3+ uptake transporter (Singh et al. 2006)
Fet5p and Fth1p of Saccharomyces cerevisiae
Fet5p (multicopper ferrooxidase) (NP_116612)
Fth1p (Fe3+ transporter) (NP_009766)
High-affinity oxidase-dependent plasma membrane Fe2+-Fe3+ uptake transporter (Singh et al., 2006)
Fio1 and Fip1 of Schizosaccharomyces pombe
Fio1 (multicopper ferrooxidase) (Q09920)
Fip1 (Fe3+ transporter) (Q09919
Iron transporter, Ftr1 (541 aas) (La Fontaine et al., 2002). FTR1, like its orthologue in S. cerevisiae (9.A.10.1.1), has an exocytoplasmic iron channeling motif and two potential iron permeation motifs in membrane-spanning regions (Terzulli and Kosman 2010).
Ftr1 of Chlamydomonas reinhardtii (AAM45938)
High affinity iron permease, CaFtr1 (Jung et al. 2008)
CaFtr1 of Cryptococcus neoformans (Q5KJQ5)
Dipartite iron uptake system, FetM (646 aas; 8 TMSs in a 1 + 7 TMS arrangement)/FetP (a periplasmic protein that enhances iron uptake by FetM) (Koch et al. 2011). FetP binds Cu2+ and Mn2+ at two different sites, 1.3 Å apart, in this homodimeric protein, and the 3-d structure with Cu2+ bound to each of the two subunits reveals different geometries at the two sites. FetMP may be an iron permease with an iron scavenging function, and possibly also an iron reducing function (Koch et al. 2011).
FetMP of E. coli
Putative heavy metal transporter, Ftr1/Tp34 or Tpd, (TP0972/TP0971). Ftr1 has 448 aas with 9 TMSs while Tp34 has 204 aas with 1 N-terminal TMSs. This antigen is a pathogen-specific membrane immunogen. This periplasmic dimeric protein binds human lactoferrin with submicromolar affinity with a stoichiometry of 2:1 (Deka et al. 2007). It's 3-d structure (1.9 Å resolution) reveals two metal binding sites per monomer with zinc bound to both sites (Deka et al. 2007).
Ftr1/Tdp of Treponema pallidum
The high-affinity oxidase-dependent iron transporter, FTR-ChpA
FTR-ChpA of Brucella melitensis
The acid-induced, low pH, ferrous iron (Fe2+) uptake transporter, EfeUOB (YcdN); cryptic in E. coli K12 (Cao et al. 2007; Grosse et al., 2006)
EfeUOB of E. coli O157:H5
EfeU - P75901
EfeO (periplasmic cupredoxin domain containing protein) - P0AB24
EfeB (peroxidase) - P31545
The Ftr1 (integral membrane transporter)/ P19 (periplasmic iron binding protein) iron uptake system. P19 has distinct copper and iron binding sites and exhibits an immunoglobulin-like fold (Chan et al., 2010).
Ftr1/P19 of Campylobacter jejuni
Ferrous iron uptake permease, EfeU
EfeU of Bacillus subtilis
High affinity Fe2+/Pb2+ permease-like protein.
Permease of Thermobaculum terrenum
Pb2+ uptake porter of 643 aas, PbtT or FTR1 (Hložková et al. 2013).
PbtT of Achromobacter xylosoxidans
Ftr1 protein of 276 aas and 8 TMSs
Ftr1 protein of Aeropyrum pernix
Siderophore-independent iron uptake porter, FtrCAB: FtrC (280 aas, 7 TMSs)/FtrA (183 aas; 1 TMS)/FtrB (110 aas, 0 TMSs) (Mathew et al. 2014). Functions with FtrD, an iron-sulfur cluster-containing ferredoxin of 465 aas.
FtrCAB of Burkholderia cenocepacia
Hypothetical protein of 208 aas
HP of Sulfolobus islandicus
Manganese ion transporter, MntH of 250 aas and 6 - 7 TMSs.
MntH of Rhizobium leguminosarum
HP of Sulfolobus acidocaldarius