Determination of transmembrane topology of the Escherichia coli natural resistance-associated macrophage protein (Nramp) ortholog

Considerations in production of the prokaryotic ZIP family transporters for structural and functional studies

Zinc ions play essential roles as components of enzymes and many other important biomolecules, and are associated with numerous diseases. The uptake of Zn2+ and other metal ions require a widely distributed transporter protein family called Zrt/Irt-like Proteins (ZIP family), the majority members of which tend to have eight transmembrane helices with both N- and C- termini located on the extracellular or periplasmic side. Their small sizes and dynamic conformations bring many difficulties in their production for structural studies either by crystallography or Cryo-EM. Here, we summarize the problems that may encounter at the various steps of processing the ZIP proteins from gene to structural and functional studies, and provide some solutions and examples from our and other labs for the cloning, expression, purification, stability screening, metal ion transport assays and structural studies of prokaryotic ZIP family transporters using Escherichia coli as a heterologous host.

Molecular Mechanism of Nramp-Family Transition Metal Transport

The Natural resistance-associated macrophage protein (Nramp) family of transition metal transporters enables uptake and trafficking of essential micronutrients that all organisms must acquire to survive. Two decades after Nramps were identified as proton-driven, voltage-dependent secondary transporters, multiple Nramp crystal structures have begun to illustrate the fine details of the transport process and provide a new framework for understanding a wealth of preexisting biochemical data. Here we review the relevant literature pertaining to Nramps' biological roles and especially their conserved molecular mechanism, including our updated understanding of conformational change, metal binding and transport, substrate selectivity, proton transport, proton-metal coupling, and voltage dependence. We ultimately describe how the Nramp family has adapted the LeuT fold common to many secondary transporters to provide selective transition-metal transport with a mechanism that deviates from the canonical model of symport.

MntC-Dependent Manganese Transport Is Essential for Staphylococcus aureus Oxidative Stress Resistance and Virulence

Work outlined in this report demonstrated that MntC-dependent manganese transport is required for S. aureus virulence. These study results support the model that MntC-specific antibodies elicited by a vaccine have the potential to disrupt S. aureus manganese transport and thus abrogate to its virulence.

Staphylococcus aureus is a human pathogen that has developed several approaches to evade the immune system, including a strategy to resist oxidative killing by phagocytes. This resistance is mediated by production of superoxide dismutase (SOD) enzymes which use manganese as a cofactor. S. aureus encodes two manganese ion transporters, MntABC and MntH, and a possible Nramp family manganese transporter, exemplified by S. aureus N315 SA1432. Their relative contributions to manganese transport have not been well defined in clinically relevant isolates. For this purpose, insertional inactivation mutations were introduced into mntC, mntH, and SA1432 individually and in combination. mntC was necessary for full resistance to methyl viologen, a compound that generates intracellular free radicals. In contrast, strains with an intact mntH gene had a minimal increase in resistance that was revealed only in mntC strains, and no change was observed upon mutation of SA1432 in strains lacking both mntC and mntH. Similarly, MntC alone was required for high cellular SOD activity. In addition, mntC strains were attenuated in a murine sepsis model. To further link these observations to manganese transport, an S. aureus MntC protein lacking manganese binding activity was designed, expressed, and purified. While circular dichroism experiments demonstrated that the secondary and tertiary structures of this protein were unaltered, a defect in manganese binding was confirmed by isothermal titration calorimetry. Unlike complementation with wild-type mntC, introduction of the manganese-binding defective allele into the chromosome of an mntC strain did not restore resistance to oxidative stress or virulence. Collectively, these results underscore the importance of MntC-dependent manganese transport in S. aureus oxidative stress resistance and virulence.

IMPORTANCE Work outlined in this report demonstrated that MntC-dependent manganese transport is required for S. aureus virulence. These study results support the model that MntC-specific antibodies elicited by a vaccine have the potential to disrupt S. aureus manganese transport and thus abrogate to its virulence.

Structural and mechanistic basis of proton-coupled metal ion transport in the SLC11/NRAMP family

Secondary active transporters of the SLC11/NRAMP family catalyse the uptake of iron and manganese into cells. These proteins are highly conserved across all kingdoms of life and thus likely share a common transport mechanism. Here we describe the structural and functional properties of the prokaryotic SLC11 transporter EcoDMT. Its crystal structure reveals a previously unknown outward-facing state of the protein family. In proteoliposomes EcoDMT mediates proton-coupled uptake of manganese at low micromolar concentrations. Mutants of residues in the transition-metal ion-binding site severely affect transport, whereas a mutation of a conserved histidine located near this site results in metal ion transport that appears uncoupled to proton transport. Combined with previous results, our study defines the conformational changes underlying transition-metal ion transport in the SLC11 family and it provides molecular insight to its coupling to protons.

Cellular uptake of transition metal ions is mediated by members of the SLC11/NRAMP family. Here the authors determine the structural and functional properties of EcoDMT, a bacterial SLC11 transporter, gathering molecular insight into its transport mechanism and proton coupling process.

Bacterial ferrous iron transport: the Feo system

To maintain iron homeostasis within the cell, bacteria have evolved various types of iron acquisition systems. Ferric iron (Fe(3+)) is the dominant species in an oxygenated environment, while ferrous iron (Fe(2+)) is more abundant under anaerobic conditions or at low pH. For organisms that must combat oxygen limitation for their everyday survival, pathways for the uptake of ferrous iron are essential. Several bacterial ferrous iron transport systems have been described; however, only the Feo system appears to be widely distributed and is exclusively dedicated to the transport of iron. In recent years, many studies have explored the role of the FeoB and FeoA proteins in ferrous iron transport and their contribution toward bacterial virulence. The three-dimensional structures for the Feo proteins have recently been determined and provide insight into the molecular details of the transport system. A highly select group of bacteria also express the FeoC protein from the same operon. This review will provide a comprehensive look at the structural and functional aspects of the Feo system. In addition, bioinformatics analyses of the feo operon and the Feo proteins have been performed to complement our understanding of this ubiquitous bacterial uptake system, providing a new outlook for future studies.

A Versatile Strategy for Production of Membrane Proteins with Diverse Topologies: Application to Investigation of Bacterial Homologues of Human Divalent Metal Ion and Nucleoside Transporters

Membrane proteins play key roles in many biological processes, from acquisition of nutrients to neurotransmission, and are targets for more than 50% of current therapeutic drugs. However, their investigation is hampered by difficulties in their production and purification on a scale suitable for structural studies. In particular, the nature and location of affinity tags introduced for the purification of recombinant membrane proteins can greatly influence their expression levels by affecting their membrane insertion. The extent of such effects typically depends on the transmembrane topologies of the proteins, which for proteins of unknown structure are usually uncertain. For example, attachment of oligohistidine tags to the periplasmic termini of membrane proteins often interferes with folding and drastically impairs expression in Escherichia coli. To circumvent this problem we have employed a novel strategy to enable the rapid production of constructs bearing a range of different affinity tags compatible with either cytoplasmic or periplasmic attachment. Tags include conventional oligohistidine tags compatible with cytoplasmic attachment and, for attachment to proteins with a periplasmic terminus, either tandem Strep-tag II sequences or oligohistidine tags fused to maltose binding protein and a signal sequence. Inclusion of cleavage sites for TEV or HRV-3C protease enables tag removal prior to crystallisation trials or a second step of purification. Together with the use of bioinformatic approaches to identify members of membrane protein families with topologies favourable to cytoplasmic tagging, this has enabled us to express and purify multiple bacterial membrane transporters. To illustrate this strategy, we describe here its use to purify bacterial homologues of human membrane proteins from the Nramp and ZIP families of divalent metal cation transporters and from the concentrative nucleoside transporter family. The proteins are expressed in E. coli in a correctly folded, functional state and can be purified in amounts suitable for structural investigations.

Penetration of three transmembrane segments of Slc11a1 in lipid bilayers

Slc11a1 is a divalent metal cation transporter with 12 putative transmembrane domains (TM) and plays a role in host defense. In present work, we investigated the secondary structure and topology of the peptides associated to Slc11a1-TM2, TM3 and TM4 (wildtype peptides and function-relating mutants) in the phospholipid vesicles (DMPC, DMPG and their mixtures) using circular dichroism, fluorescence spectroscopy and differential scanning calorimetry. We found that TM3 is obviously different in secondary structure and topology from TM2 to TM4 in the lipid membranes. The peptide TM3 is less structured and embedded in the lipid membranes less deeply than TM2 and TM4 at pH 5.5 and 7. The insertion position of TM3 in the lipid membranes is adjusted by pH, more deeply at more acidic pH environment, whereas the locations of TM2 and TM4 in the lipid membranes are less changed with pH. The E139A substitution of TM3 significantly impairs the pH dependence of the buried depth of TM3 and causes a pronounced increase in helicity in all DMPG-containing lipid vesicles at pH 5.5 and 7 and in DMPC at pH 4. In contrast, TM2 and TM4 are similar in topology. The G169D mutation has little effect on the topological arrangement of TM4 in membranes. The property of headgroups of the phospholipids has an effect on the secondary structure and topology of the peptides. All peptides could be structured with more helicity and embedded more deeply in DMPG-containing lipid vesicles than in DMPC membrane at pH 5.5 and 7.

Computational comparative study of tuberculosis proteomes using a model learned from signal peptide structures

Secretome analysis is important in pathogen studies. A fundamental and convenient way to identify secreted proteins is to first predict signal peptides, which are essential for protein secretion. However, signal peptides are highly complex functional sequences that are easily confused with transmembrane domains. Such confusion would obviously affect the discovery of secreted proteins. Transmembrane proteins are important drug targets, but very few transmembrane protein structures have been determined experimentally; hence, prediction of the structures is essential. In the field of structure prediction, researchers do not make assumptions about organisms, so there is a need for a general signal peptide predictor.To improve signal peptide prediction without prior knowledge of the associated organisms, we present a machine-learning method, called SVMSignal, which uses biochemical properties as features, as well as features acquired from a novel encoding, to capture biochemical profile patterns for learning the structures of signal peptides directly.We tested SVMSignal and five popular methods on two benchmark datasets from the SPdb and UniProt/Swiss-Prot databases, respectively. Although SVMSignal was trained on an old dataset, it performed well, and the results demonstrate that learning the structures of signal peptides directly is a promising approach. We also utilized SVMSignal to analyze proteomes in the entire HAMAP microbial database. Finally, we conducted a comparative study of secretome analysis on seven tuberculosis-related strains selected from the HAMAP database. We identified ten potential secreted proteins, two of which are drug resistant and four are potential transmembrane proteins.SVMSignal is publicly available at http://bio-cluster.iis.sinica.edu.tw/SVMSignal. It provides user-friendly interfaces and visualizations, and the prediction results are available for download.

Nutritional immunity: homology modeling of Nramp metal import

The Natural resistance-associated macrophage proteins (Nramp1 and 2) are proton-dependent solute carriers of divalent metals such as Fe(2+) and Mn(2+) (Slc11a1 and 2). Their expression in both resting and microbicidal macrophages which metabolize iron differently, raises questions about Nramp mechanism of Me(2+) transport and its impact in distinct phenotypic contexts. We developed a low resolution 3D model for Slc11 based on detailed phylogeny and remote homology threading using Escherichia coli Nramp homolog (proton-dependent Mn(2+) transporter, MntH) as experimental system. The predicted fold is consistent with determinations of transmembrane topology and activity; it indicates Slc11 carriers are part of the LeuT superfamily. Homology implies that inverted structural symmetry facilitates Slc11 H(+)-driven Me(2+) import and provides a 3D framework to test structure-activity relationships in macrophages and study functional evolution of MntH/Nramp (Slc11) carriers.

The phage shock protein PspA facilitates divalent metal transport and is required for virulence of Salmonella enterica sv. Typhimurium

The phage shock protein (Psp) system is induced by extracytoplasmic stress and thought to be important for the maintenance of proton motive force. We investigated the contribution of PspA to Salmonella virulence. A pspA deletion mutation significantly attenuates the virulence of Salmonella enterica serovar Typhimurium following intraperitoneal inoculation of C3H/HeN (Ity(r) ) mice. PspA was found to be specifically required for virulence in mice expressing the natural resistance-associated macrophage protein 1 (Nramp1) (Slc11a1) divalent metal transporter, which restricts microbial growth by limiting the availability of essential divalent metals within the phagosome. Salmonella competes with Nramp1 by expressing multiple metal uptake systems including the Nramp-homologue MntH, the ABC transporter SitABCD and the ZIP family transporter ZupT. PspA was found to facilitate Mn(2+) transport by MntH and SitABCD, as well as Zn(2+) and Mn(2+) transport by ZupT. In vitro uptake of (54) Mn(2+) by MntH and ZupT was reduced in the absence of PspA. Transport-deficient mutants exhibit reduced viability in the absence of PspA when grown under metal-limited conditions. Moreover, the ZupT transporter is required for Salmonella enterica serovar Typhimurium virulence in Nramp1-expressing mice. We propose that PspA promotes Salmonella virulence by maintaining proton motive force, which is required for the function of multiple transporters mediating bacterial divalent metal acquisition during infection.

Evidence that highly conserved residues of transmembrane segment 6 of Escherichia coli MntH are important for transport activity

Nramp (natural resistance-associated macrophage protein) family members have been characterized in mammals, yeast, and bacteria as divalent metal ion/H(+) symporters. In previous work, a bioinformatic approach was used for the identification of residues that are conserved within the Nramp family [Haemig, H. A., and Brooker, R. J. (2004) J. Membr. Biol. 201 (2), 97-107]. On the basis of site-directed mutagenesis of highly conserved negatively charged residues, a model was proposed for the metal binding site of the Escherichia coli homologue, MntH. In this study, we have focused on the highly conserved residues, including two histidines, of transmembrane segment 6 (TMS-6). Multiple mutants were made at the eight conserved sites (i.e., Gly-205, Ala-206, Met-209, Pro-210, His-211, Leu-215, His-216, and Ser-217) in TMS-6 of E. coli MntH. Double mutants involving His-211 and His-216 were also created. The results indicate the side chain volume of these residues is critically important for function. In most cases, only substitutions that are closest in side chain volume still permit transport. In addition, the K(m) for metal binding is largely unaffected by mutations in TMS-6, whereas V(max) values were decreased in all mutants characterized kinetically. Thus, these residues do not appear to play a role in metal binding. Instead, they may comprise an important face on TMS-6 that is critical for protein conformational changes during transport. Also, in contrast to other studies, our data do not strongly indicate that the conserved histidine residues play a role in the pH regulation of metal transport.

Transmembrane topology of the mammalian Slc11a2 iron transporter

The mammalian Slc11a1 and Slc11a2 proteins define a large family of secondary metal transporters. Slc11a1 and Slc11a2 function as pH-dependent divalent cation transporters that play a critical role in host defenses against infections and in Fe²⁺ homeostasis, respectively. The position and polarity of individual transmembrane domains (TMD) of Slc11a2 were studied by an epitope tagging method based on the insertion of small antigenic hemagglutinin A (HA) peptides (YPYDVPDYAS) in predicted intra- or extracellular loops of the protein. The tagged proteins were expressed in transfected LLC-PK1 kidney cells and tested for transport activity, and the polarity of inserted tags with respect to the plasma membrane was determined by immunofluorescence in intact and permeabilized cells. HA epitope tags were inserted at positions 1, 98, 131, 175, 201, 243, 284, 344, 403, 432, 468, 504, and 561. Insertions at positions 98, 131, 175, 403, and 432 abrogated metal transport by Slc11a2, while insertions at positions 1, 201, 243, 284, 344, 468, 504, and 561 resulted in functional proteins. Topology mapping in functional HA-tagged Slc11a2 proteins indicated that the N-terminus (1), as well as loops delineated by TMD4−5 (201), TMD6−7 (284), and TMD10−11 (468), and C-terminus (561) are intracellular, while loops separating TMD5−6 (243), TMD7−8 (344), and TMD11−12 (504) are extracellular. These results are compatible with a topology of 12 transmembrane domains, with intracellular amino and carboxy termini. Structural models constructed by homology threading support this 12TMD topology and show 2-fold structural symmetry in the arrangement of membrane helices for TM1−5 and TM6−10 (conserved Slc11 hydrophobic core).

Solute carrier 11 cation symport requires distinct residues in transmembrane helices 1 and 6

Ubiquitous solute carriers 11 (SLC11) contribute to metal-ion homeostasis by importing Me(2+) and H(+) into the cytoplasm. To identify residues mediating cation symport, Escherichia coli proton-dependent manganese transporter (MntH) was mutated at five SLC11-specific transmembrane (TM) sites; each mutant activity was compared with wild-type MntH, and the biochemical results were tested by homology threading. Cd(2+) and H(+) uptake kinetics were analyzed in whole cells as a function of pH and temperature, and right-side out membrane vesicles were used to detail energy requirements and to probe site accessibility by Cys replacement and thiol modification. This approach revealed that TM segment 1 (TMS1) residue Asp(34) couples H(+) and Me(2+) symport and contributes to MntH forward transport electrogenicity, whereas the TMS6 His(211) residue mediates pH-dependent Me(2+) uptake; MntH Asn(37), Asn(250), and Asn(401) in TMS1, TMS7, and TMS11 participate in transporter cycling and/or helix packing interactions. These biochemical results fit the LeuT/SLC6 structural fold, which suggests that conserved peptide motifs Asp(34)-Pro-Gly (TMS1) and Met-Pro-His(211) (TMS6) form antiparallel "TM helix/extended peptide" boundaries, lining a "pore" cavity and enabling H(+)-dependent Me(2+) import.

Metallosensors, the ups and downs of gene regulation

In fungal cells, transcriptional regulatory mechanisms play a central role in both the homeostatic regulation of the essential metals iron, copper and zinc and in the detoxification of heavy metal ions such as cadmium. Fungi detect changes in metal ion levels using unique metallo-regulatory factors whose activity is responsive to the cellular metal ion status. New studies have revealed that these factors not only regulate the expression of genes required for metal ion acquisition, storage or detoxification but also globally remodel metabolism to conserve metal ions or protect against metal toxicity. This review focuses on the mechanisms metallo-regulators use to up- and down-regulate gene expression.

Topology-informed strategies for the overexpression and purification of membrane proteins

Membrane proteins represent a significant fraction of all genomes and play key roles in many aspects of biology, but their structural analysis has been hampered by difficulties in large-scale production and crystallisation. To overcome the first of these hurdles, we present here a systematic approach for expression and affinity-tagging which takes into account transmembrane topology. Using a set of bacterial transporters with known topologies, we tested the efficacy of a panel of conventional and Gateway recombinational cloning vectors designed for protein expression under the control of the tac promoter, and for the addition of differing N- and C-terminal affinity tags. For transporters in which both termini are cytoplasmic, C-terminal oligohistidine tagging by recombinational cloning typically yielded functional protein at levels equivalent to or greater than those achieved by conventional cloning. In contrast, it was not effective for examples of the substantial minority of proteins that have one or both termini located on the periplasmic side of the membrane, possibly because of impairment of membrane insertion by the tag and/or att-site-encoded sequences. However, fusion either of an oligohistidine tag to cytoplasmic (but not periplasmic) termini, or of a Strep-tag II peptide to periplasmic termini using conventional cloning vectors did not interfere with membrane insertion, enabling high-level expression of such proteins. In conjunction with use of a C-terminal Lumio fluorescence tag, which we found to be compatible with both periplasmic and cytoplasmic locations, these findings offer a system for strategic planning of construct design for high throughput expression of membrane proteins for structural genomics projects.

Recent progress in structure-function analyses of Nramp proton-dependent metal-ion transporters

The natural resistance-associated macrophage protein (Nramp) homologs form a family of proton-coupled transporters that facilitate the cellular absorption of divalent metal ions (Me2+, including Mn2+, Fe2+, Co2+, and Cd2+). The Nramp, or solute carrier 11 (SLC11), family is conserved in eukaryotes and bacteria. Humans and rodents express 2 parologous genes that are associated with iron disorders and immune diseases. The NRAMP1 (SLC11A1) protein is specific to professional phagocytes and extrudes Me2+ from the phagosome to defend against ingested microbes; polymorphisms in the NRAMP1 gene are associated with various immune diseases. Several isoforms of NRAMP2 (SLC11A2, DMT1, DCT1) are expressed ubiquitously in recycling endosomes or specifically at the apical membrane of epithelial cells in intestine and kidneys, and can contribute to iron overload, whereas mutations impairing NRAMP2 function cause a form of congenital microcytic hypochromic anemia. Structure-function studies, using various experimental models, and mutagenesis approaches have begun to reveal the overall transmembrane organization of Nramp, some of the transmembrane segments (TMS) that are functionally important, and an unusual mechanism coupling Me2+ and proton H+ transport. The approaches used include functional complementation of yeast knockout strains, electrophysiology analyses in Xenopus oocytes, and transport assays that use mammalian and bacterial cells and direct and indirect measurements of SLC11 transporter properties. These complementary studies enabled the identification of TMS1 and 6 as crucial structural segments for Me2+ and H+ symport, and will help develop a deeper understanding of the Nramp transport mechanism and its contribution to Me2+ homeostasis in human health and diseases.

Manganese transport and the role of manganese in virulence

Two areas of research have recently converged to highlight important roles for Mn(2+) in pathogenesis: the recognition that both bacterial Nramp homologs and members of LraI family of proteins are Mn(2+) transporters. Their mutation is associated with decreased virulence of various bacterial species. Thus, Mn(2+) appears to be essential for bacterial virulence. This review describes what is currently known about Mn(2+) transport in prokaryotes and how prokaryotic Mn(2+) transport is regulated. Some of the phenotypes that arise when microorganisms lack Mn(2+) are then discussed, with an emphasis on those phenotypes involving pathogenesis. The concluding section describes possible enzymatic roles for Mn(2+) that might help explain why Mn(2+) is necessary for virulence.

The NRAMP family of metal-ion transporters

The family of NRAMP metal ion transporters functions in diverse organisms from bacteria to human. NRAMP1 functions in metal transport across the phagosomal membrane of macrophages, and defective NRAMP1 causes sensitivity to several intracellular pathogens. DCT1 (NRAMP2) transport metal ions at the plasma membrane of cells of both the duodenum and in peripheral tissues, and defective DCT1 cause anemia. The driving force for the metal-ion transport is proton gradient (protonmotive force). In DCT1 the stoichiometry between metal ion and proton varied at different conditions due to a mechanistic proton slip. Though the metal ion transport by Smf1p, the yeast homolog of DCT1, is also a protonmotive force, a slippage of sodium ions was observed. The mechanism of the above phenomena could be explained by a combination between transporter and channel mechanisms.

Iron Transport in Escherichia Coli: All has not been said and Done

During recent years new systems involved in iron transport were identified in the old workhorse Escherichia coli (and in other enterobacteria). This came as a bit of a surprise because one might think transport of this essential trace element was already thoroughly studied. Moreover, it appears that iron homeostasis consists not only of uptake but also of efflux of this potentially toxic redox-active metal. New findings in E. coli will be discussed and compared to the situation in other bacteria.

Modulation of Iron Availability at the Host-Pathogen Interface in Phagocytic Cells

This review summarizes recent data on iron metabolism in macrophages, with a special emphasis on possible bacteriostatic and bactericidal consequences for intracellular pathogens. It includes the role of biological chelators and transporters in normal macrophage physiology and antimicrobial defense. Iron is an essential metal cofactor for many biochemical pathways in mammals. However, excess iron promotes the formation of cytotoxic oxygen derivatives so that systemic iron levels must be tightly regulated. The mechanism of iron recycling by macrophages including iron efflux from erythrocyte-containing phagosomes, iron release from macrophages, and entry into the transferrin (Tf) cycle remain poorly understood. Ferroportin expression in the liver, spleen, and bone marrow cells appears to be restricted to macrophages. Mutant mice bearing a conditional deletion of the ferroportin gene in macrophages show retention of iron by hepatic Kupffer cells and splenic macrophages. Hepcidin is induced by lipopolysaccharide (LPS) in mouse spleens and splenic macrophage in vitro and appears to mediate the LPS-induced down-regulation of ferroportin in the intestine and in splenic macrophages, suggesting that inflammatory agents may regulate iron metabolism through modulation of ferroportin expression. The host transporter Nramp1 may compete directly with bacterial divalent-metal transport systems for the acquisition of divalent metals within the phagosomal space. The ultimate outcome of these competing interactions influences the ability of pathogens to survive and replicate intracellularly. This seems particularly relevant to the Salmonella, Leishmania, and Mycobacterium spp., in which inactivating mutations in Nramp1 abrogate the natural resistance of macrophages to these pathogens.

Importance of conserved acidic residues in mntH, the Nramp homolog of Escherichia coli

A bioinformatic approach was used for the identification of residues that are conserved within the Nramp family of metal transporters. Site-directed mutagenesis was then carried out to change six conserved acidic residues (i.e., Asp-34, Glu-102, Asp-109, Glu-112, Glu-154, and Asp-238) in the E. coli Nramp homolog mntH. Of these six, five of them, Asp-34, Glu-102, Asp-109, Glu-112, and Asp-238 appear to be important for function since conservative substitutions at these sites result in a substantial loss of transport function. In addition, all of the residues within the signature sequence of the Nramp family, DPGN, were also mutated in this study. Each residue was changed to several different side chains, and of ten site-directed mutations made in this motif, only P35G showed any measurable level of (54)Mn(2+) uptake with a V(max) value of approximately 10% of wild-type and a slightly elevated K(m) value. Overall, the data are consistent with a model where helix breakers in the conserved DPGN motif in TMS-1 provide a binding pocket in which Asp-34, Asn-37, Asp-109, Glu-112 (and possibly other residues) are involved in the coordination of Mn(2+). Other residues such as Glu-102 and Asp238 may play a role in the release of Mn(2+) to the cytoplasm or may be involved in maintaining secondary structure.

The Nramp orthologue of Cryptococcus neoformans is a pH-dependent transporter of manganese, iron, cobalt and nickel

Cryptococcus neoformans is an important human opportunistic pathogen and a facultative intracellular parasite, particularly in HIV-infected individuals. Little is known about metal ion transport in this organism. C. neoformans encodes a single member of the Nramp (natural resistance-associated macrophage protein) family of bivalent cation transporters, known as Cramp, which we have cloned and expressed in Xenopus laevis oocytes and Spodoptera frugiperda Sf 21 insect cells. Cramp induces saturable transport of a broad range of bivalent transition series cations, including Mn2+, Fe2+, Co2+ and Ni2+. Maximal cation transport occurs at pH 5.5-6.0, consistent with the proton gradient-based energetics of other Nramp orthologues. Mn2+ transport is diminished in the presence of 140 mM Na+, compatible with a Na+ slippage mechanism proposed for the Saccharomyces cerevisiae Nramp orthologue Smf1p. Cramp resembles Smf1p with respect to predicted membrane topology, substrate specificity and pH dependence, but differs in terms of its apparent affinity for Mn2+ and negligible inhibition by Zn2+. Cramp is the first Nramp orthologue from a fungal pathogen to be functionally characterized. Insights afforded by these findings will allow the formulation of new hypotheses regarding the role of metal ions in the pathophysiology of cryptococcosis.

Independent mutations in mouse Vangl2 that cause neural tube defects in looptail mice impair interaction with members of the Dishevelled family

Mammalian Vangl1 and Vangl2 are highly conserved membrane proteins that have evolved from a single ancestral protein Strabismus/Van Gogh found in Drosophila. Mutations in the Vangl2 gene cause a neural tube defect (craniorachischisis) characteristic of the looptail (Lp) mouse. Studies in model organisms indicate that Vangl proteins play a key developmental role in establishing planar cell polarity (PCP) and in regulating convergent extension (CE) movements during embryogenesis. The role of Vangl1 in these processes is virtually unknown, and the molecular function of Vangl1 and Vangl2 in PCP and CE is poorly understood. Using a yeast two-hybrid system, glutathione S-transferase pull-down and co-immunoprecipitation assays, we show that both mouse Vangl1 and Vangl2 physically interact with the three members of the cytoplasmic Dishevelled (Dvl) protein family. This interaction is shown to require both the predicted cytoplasmic C-terminal half of Vangl1/2 and a portion of the Dvl protein containing PDZ and DIX domains. In addition, we show that the two known Vangl2 loss-of-function mutations identified in two independent Lp alleles associated with neural tube defects impair binding to Dvl1, Dvl2, and Dvl3. These findings suggest a molecular mechanism for the neural tube defect seen in Lp mice. Our observations indicate that Vangl1 biochemical properties parallel those of Vangl2 and that Vangl1 might, therefore, participate in PCP and CE either in concert with Vangl2 or independently of Vangl2 in discrete cell types.