Cloning of Pichia pastoris Fet3: insights into the high affinity iron uptake system

The Rhizophagus irregularis permease RiFTR1 functions without a ferroxidase partner for reductive iron transport

The contribution of arbuscular mycorrhizal fungi (AM fungi) to plant iron (Fe) acquisition has been demonstrated in several studies. A previous investigation revealed that the AM fungus Rhizophagus irregularis utilizes a high-affinity reductive pathway for Fe uptake, mediated by the Fe transporter RiFTR1. In this study, we used a genome-wide approach in R. irregularis to find genes encoding ferroxidases of the multicopper oxidase (MCO) gene family in an attempt to identify the ferroxidase partner of RiFTR1. Nine genes putatively encoding MCOs (RiMCO1-9) were identified. Yeast complementation assays demonstrated that RiMCO1 and RiMCO3 can function as ferroxidases, suggesting their involvement in the reductive Fe uptake pathway. Surprisingly, RiFTR1 was capable of transporting Fe in yeast without a ferroxidase partner, resembling the Fe transport mechanism of plant IRT1-like systems. RiFTR1 exhibited increase expression in arbuscules. Overexpression of RiFTR1 in Medicago truncatula roots led to enhanced mycorrhizal colonization and arbuscule abundance, highlighting the significance of Fe for AM symbiosis.

The Ferroxidase-Permease System for Transport of Iron Across Membranes: From Yeast to Humans

Transport of iron across the cell membrane is a tightly controlled process carried out by specific proteins in all living cells. In yeast and in mammals, a system formed by an enzyme with ferroxidase activity coupled to a membrane transporter supports iron uptake or iron efflux, respectively. Ferroxidase belongs to the family of blue multicopper oxidases, enzymes able to couple the one-electron oxidation of substrate(s) to full reduction of molecular oxygen to water. On the other hand, the permeases are widely different and are specific to Fe3+ and Fe2+ in yeast and multicellular organisms, respectively. This review will describe the yeast and human ferroxidase-permease systems, highlighting similarities and differences in structure, function and regulation of the respective protein components.

Pichia pastoris Fep1 is a [2Fe-2S] protein with a Zn finger that displays an unusual oxygen-dependent role in cluster binding

Fep1, the iron-responsive GATA factor from the methylotrophic yeast Pichia pastoris, has been characterised both in vivo and in vitro. This protein has two Cys2-Cys2 type zinc fingers and a set of four conserved cysteines arranged in a Cys-X5-Cys-X8-Cys-X2-Cys motif located between the two zinc fingers. Electronic absorption and resonance Raman spectroscopic analyses in anaerobic and aerobic conditions indicate that Fep1 binds iron in the form of a [2Fe-2S] cluster. Site-directed mutagenesis shows that replacement of the four cysteines with serine inactivates this transcriptional repressor. Unexpectedly, the inactive mutant is still able to bind a [2Fe-2S] cluster, employing two cysteine residues belonging to the first zinc finger. These two cysteine residues can act as alternative cluster ligands selectively in aerobically purified Fep1 wild type, suggesting that oxygen could play a role in Fep1 function by causing differential localization of the [Fe-S] cluster.

Secretion of two novel enzymes, manganese 9S-lipoxygenase and epoxy alcohol synthase, by the rice pathogen Magnaporthe salvinii

The mycelium of the rice stem pathogen, Magnaporthe salvinii, secreted linoleate 9S-lipoxygenase (9S-LOX) and epoxy alcohol synthase (EAS). The EAS rapidly transformed 9S-hydroperoxy-octadeca-10E,12Z-dienoic acid (9S-HPODE) to threo 10 (11)-epoxy-9S-hydroxy-12Z-octadecenoic acid, but other hydroperoxy FAs were poor substrates. 9S-LOX was expressed in Pichia pastoris. Recombinant 9S-LOX oxidized 18:2n-6 directly to 9S-HPODE, the end product, and also to two intermediates, 11S-hydroperoxy-9Z,12Z-octadecenoic acid (11S-HPODE; ∼5%) and 13R-hydroperoxy-9Z,11E-octadecadienoic acid (13R-HPODE; ∼1%). 11S- and 13R-HPODE were isomerized to 9S-HPODE, probably after oxidation to peroxyl radicals, β-fragmentation, and oxygen insertion at C-9. The 18:3n-3 was oxidized at C-9, C-11, and C-13, and to 9,16-dihydroxy-10E,12,14E-octadecatrienoic acid. 9S-LOX contained catalytic manganese (Mn:protein ∼0.2:1; Mn/Fe, 1:0.05), and its sequence could be aligned with 77% identity to 13R-LOX with catalytic manganese lipoxygenase (13R-MnLOX) of the Take-all fungus. The Leu350Met mutant of 9S-LOX shifted oxidation of 18:2n-6 from C-9 to C-13, and the Phe347Leu, Phe347Val, and Phe347Ala mutants of 13R-MnLOX from C-13 to C-9. In conclusion, M. salvinii secretes 9S-LOX with catalytic manganese along with a specific EAS. Alterations in the Sloane determinant of 9S-LOX and 13R-MnLOX with larger and smaller hydrophobic residues interconverted the regiospecific oxidation of 18:2n-6, presumably by altering the substrate position in relation to oxygen insertion.

Isolation and characterisation of EfeM, a periplasmic component of the putative EfeUOBM iron transporter of Pseudomonas syringae pv. syringae

The EfeM protein is a component of the putative EfeUOBM iron-transporter of Pseudomonas syringae pathovar syringae and is thought to act as a periplasmic, ferrous-iron binding protein. It contains a signal peptide of 34 amino acid residues and a C-terminal 'Peptidase_M75' domain of 251 residues. The C-terminal domain contains a highly conserved 'HXXE' motif thought to act as part of a divalent cation-binding site. In this work, the gene (efeM or 'Psyr_3370') encoding EfeM was cloned and over-expressed in Escherichia coli, and the mature protein was purified from the periplasm. Mass spectrometry confirmed the identity of the protein (M(W) 27,772Da). Circular dichroism spectroscopy of EfeM indicated a mainly alpha-helical structure, consistent with bioinformatic predictions. Purified EfeM was crystallised by hanging-drop vapor diffusion to give needle-shaped crystals that diffracted to a resolution of 1.6A. This is the first molecular study of a peptidase M75 domain with a presumed iron transport role.

EfeO-cupredoxins: major new members of the cupredoxin superfamily with roles in bacterial iron transport

The EfeUOB system of Escherichia coli is a tripartite, low pH, ferrous iron transporter. It resembles the high-affinity iron transporter (Ftr1p-Fet3p) of yeast in that EfeU is homologous to Ftr1p, an integral-membrane iron-permease. However, EfeUOB lacks an equivalent of the Fet3p component--the multicopper oxidase with three cupredoxin-like domains. EfeO and EfeB are periplasmic but their precise roles are unclear. EfeO consists primarily of a C-terminal peptidase-M75 domain with a conserved 'HxxE' motif potentially involved in metal binding. The smaller N-terminal domain (EfeO-N) is predicted to be cupredoxin (Cup) like, suggesting a previously unrecognised similarity between EfeO and Fet3p. Our structural modelling of the E. coli EfeO Cup domain identifies two potential metal-binding sites. Site I is predicted to bind Cu(2+) using three conserved residues (C41 and 103, and E66) and M101. Of these, only one (C103) is conserved in classical cupredoxins where it also acts as a Cu ligand. Site II most probably binds Fe(3+) and consists of four well conserved surface Glu residues. Phylogenetic analysis indicates that the EfeO-Cup domains form a novel Cup family, designated the 'EfeO-Cup' family. Structural modelling of two other representative EfeO-Cup domains indicates that different subfamilies employ distinct ligand sets at their proposed metal-binding sites. The ~100 efeO homologues in the bacterial sequence databases are all associated with various iron-transport related genes indicating a common role for EfeO-Cup proteins in iron transport, supporting a new copper-iron connection in biology.

Cloning and characterization of the genes encoding the high-affinity iron-uptake protein complex Fet3/Ftr1 in the basidiomycete Phanerochaete chrysosporium

MCO1, a multicopper oxidase from Phanerochaete chrysosporium exhibiting strong ferroxidase activity, has recently been described. This enzyme shows biochemical and structural similarities with the yeast Fet3p, a type I membrane glycoprotein that efficiently oxidizes Fe(II) to Fe(III) for its subsequent transport to the intracellular compartment by the iron permease Ftr1p. The genome database of P. chrysosporium was searched to verify whether it includes a canonical fet3 in addition to mco1, and single copies of fet3 and ftr1 orthologues were found, separated by a divergent promoter. Pc-fet3 encodes a 628 aa protein that exhibits overall identities of about 40 % with other reported Fet3 proteins. In addition to a secretion signal, it has a C-terminal transmembrane domain, characteristic of these cell-surface-attached ferroxidases. Structural modelling of Pc-Fet3 revealed that the active site has all the residues known to be essential for ferroxidase activity. Pc-ftr1 encodes a 393 aa protein that shows about 38 % identity with several Ftr1 proteins from ascomycetes. Northern hybridization studies showed that the mRNA levels of both genes are reduced upon supplementation of the growth medium with iron, supporting the functional coupling of Fet3 and Ftr1 proteins in vivo.

A GATA-type transcription factor regulates expression of the high-affinity iron uptake system in the methylotrophic yeast Pichia pastoris

The ferroxidase Fet3 and the permease Ftr1 constitute a well-conserved high-affinity iron uptake system in yeast. We have investigated the mechanism of transcriptional regulation of Fet3 in the methylotrophic yeast Pichia pastoris. Isolation and functional analysis of the Fet3 promoter indicate that a GATA sequence element plays a role in iron-dependent expression of Fet3. A GATA-type transcription factor, which we have named Fep1, has been partially cloned and it is shown to belong to the family of iron-responsive fungal GATA-factors. These factors share the presence of two Cys(2)-Cys(2) zinc-finger motifs and a set of four conserved cysteines, and are involved in the regulation of siderophore biosynthesis and/or high-affinity iron uptake. Disruption of the FEP1 gene in P. pastoris leads to constitutively high expression of Fet3, irrespective of iron levels, indicating that Fep1 is a transcriptional repressor. EMSA analyses evidence that Fep1 binds to DNA only in the presence of iron.

EfeUOB (YcdNOB) is a tripartite, acid-induced and CpxAR-regulated, low-pH Fe2+ transporter that is cryptic in Escherichia coli K-12 but functional in E. coli O157:H7

Escherichia coli possesses iron transporters specific for either Fe2+ or Fe3+. Although Fe2+ is far more soluble than Fe3+, it rapidly oxidizes aerobically at pH > or = 7. Thus, FeoAB, the major Fe2+ transporter of E. coli, operates anaerobically. However, Fe2+ remains stable aerobically under acidic conditions, although a low-pH Fe2+ importer has not been previously identified. Here we show that ycdNOB (efeUOB) specifies the first such transporter. efeUOB is repressed at high pH by CpxAR, and is Fe2+-Fur repressed. EfeU is homologous to the high-affinity iron permease, Ftr1p, of Saccharomyces cerevisiae and other fungi. EfeO is periplasmic with a cupredoxin N-terminal domain; EfeB is also periplasmic and is haem peroxidase-like. All three Efe proteins are required for Efe function. The efeU gene of E. coli K-12 is cryptic due to a frameshift mutation - repair of the single-base-pair deletion generates a functional EfeUOB system. In contrast, the efeUOB operon of the enterohaemorrhagic strain, O157:H7, lacks any frameshift and is functional. A 'wild-type' K-12 strain bearing a functional EfeUOB displays a major growth advantage under aerobic, low-pH, low-iron conditions when a competing metal is provided. 55Fe transport assays confirm the ferrous iron specificity of EfeUOB.

The Iron/Lead Transporter Superfamily of Fe3+/Pb2+ Uptake Systems

Oxidase-dependent ferrous iron uptake transporters of the OFeT family and lead uptake transporters of the PbrT family comprise the iron/lead transporter (ILT) superfamily (transporter classification No. 9.A.10). All sequenced homologues of the ILT superfamily were multiply aligned, and conserved motifs, including fully conserved acidic residues in putative transmembrane segments (TMSs) 1 and 4, previously implicated in heavy metal binding, were identified. 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. The results serve to characterize and provide evolutionary insight into a novel superfamily of heavy metal uptake transporters.

The yeast multicopper oxidase Fet3p and the iron permease Ftr1p physically interact

High affinity iron uptake in yeast is carried out by a multicomponent system formed by the ferroxidase Fet3p and the iron permease Ftr1p. The currently accepted model predicts that Fet3p and Ftr1p are functionally associated, however, a structural interaction between these two proteins has not been proven yet. The methylotrophic yeast Pichia pastoris has been used to perform cross-linking studies aimed to demonstrate the existence of a Fet3p-Ftr1p complex. Cross-linking of membrane suspensions with the membrane-impermeable reagents DTSSP and BS(3) has evidenced the presence of a high molecular weight band with Fet3p oxidase activity. This band has been purified and subjected to N-terminal sequence analysis. Two sequences were found in the cross-linked species, one of which could be assigned to Fet3p and the other to Ftr1p. This is the first experimental demonstration that Fet3p and Ftr1p are physically associated.

Post-translational modifications of the AFET3 gene product: a component of the iron transport system in budding cells and mycelia of the yeast Arxula adeninivorans

The yeast Arxula adeninivorans is characterized by a temperature-dependent dimorphism. A. adeninivorans grows as budding cells at temperatures up to 42 degrees C, but forms mycelia at higher temperatures. A strong correlation exists between morphological status and iron uptake, achieved by two transport systems that differ in iron affinity. In the presence of high Fe(II) concentrations (>2 microm), budding cells accumulate iron concentrations up to seven-fold higher than those observed in mycelia, while at low Fe(II) concentrations (<2 microm), both cell types accumulate similar amounts of iron. The copper-dependent Fe(II) oxidase Afet3p, composed of 615 amino acids, is a component of the high-affinity iron transport system. This protein shares a high degree of homology with other yeast iron transport proteins, namely Fet3p of Saccharomyces cerevisiae, Cafet3p of Candida albicans and Pfet3p of Pichia pastoris. Expression of the AFET3 gene is found to be strongly dependent on iron concentration but independent of the morphological stage; however, cell morphology was found to influence post-translational modifications of the gene product. O-glycosylation was observed in budding cells only, whereas N-glycosylation occurred in both cell types. The N-glycosylated 103 kDa glycoprotein matures into the 108.5 kDa form, further characterized by serine phosphorylation. Both N-glycosylation and phosphorylation occur at low iron concentrations (< or =5 microm). The mature Afet3p of 108.5 kDa is uniformly distributed within the plasma membrane in cells of both morphological stages.

Genes for multicopper proteins and laccase activity: common features in plant-associatedFusariumisolates

Eight Fusarium isolates from diverse plant habitats produced laccase activities when cultured on low-carbon medium. Three sequences with high homology to laccase genes were detected in the genome of Fusarium proliferatum, NRRL 31071, an opportunistic pathogen of wheat (Triticum spp.). The sequences aligned with laccase genes from Ascomycetes, whereas genes from other fungal taxonomic groups and from plants grouped separately. Sequences for related multicopper oxidases with ascorbate oxidase activity were also separated discretely. A fourth sequence from NRRL 31071 had highest homology to genes for multicopper proteins associated with ferrooxidase activities in yeasts. Each of the four sequences had high homology to sequences within the genome of a ligninolytic F. proliferatum from forest soil. RFLP analysis of the genomes of three other F. proliferatum isolates and a phylogenetically closely related isolate, Fusarium fujikuroi, showed that fragments hybridizing with the four multicopper oxidase genes were similar in size to those from the NRRL 31071 genome. Weaker hybridization was observed with the phylogenetically more distant Fusarium anthophilum. No hybridization was observed with an isolate of Fusarium culmorum, a wheat pathogen, from a different section of the genus Fusarium, although this isolate produced laccases.Key words: ferrooxidase, Fusarium proliferatum, laccase, multicopper oxidase.

Mutational analysis of the iron binding site of Saccharomyces cerevisiae ferroxidase Fet3. An in vivo study

The role of residues predicted to be involved in the binding of iron by the yeast ferroxidase Fet3 has been studied by site-directed mutagenesis. The effect of Fet3 mutations E185A, E185Q, Y354F, D409V and H489D has been investigated in vivo by kinetic analyses of high affinity iron uptake. Our results indicate that Glu-185 is critical for the binding of iron, since substitution of this residue with Ala or Gln strongly affects both growth and the kinetic parameters of high affinity iron uptake, greatly increasing K(m). Mutations Y354F and D409V result in less severe alteration of high affinity iron uptake, while mutant H489D is unable to grow under conditions of iron limitation.