Crystallography & NMR system: A new software suite for macromolecular structure determination

A new software suite, called Crystallography & NMR System (CNS), has been developed for macromolecular structure determination by X-ray crystallography or solution nuclear magnetic resonance (NMR) spectroscopy. In contrast to existing structure-determination programs, the architecture of CNS is highly flexible, allowing for extension to other structure-determination methods, such as electron microscopy and solid-state NMR spectroscopy. CNS has a hierarchical structure: a high-level hypertext markup language (HTML) user interface, task-oriented user input files, module files, a symbolic structure-determination language (CNS language), and low-level source code. Each layer is accessible to the user. The novice user may just use the HTML interface, while the more advanced user may use any of the other layers. The source code will be distributed, thus source-code modification is possible. The CNS language is sufficiently powerful and flexible that many new algorithms can be easily implemented in the CNS language without changes to the source code. The CNS language allows the user to perform operations on data structures, such as structure factors, electron-density maps, and atomic properties. The power of the CNS language has been demonstrated by the implementation of a comprehensive set of crystallographic procedures for phasing, density modification and refinement. User-friendly task-oriented input files are available for nearly all aspects of macromolecular structure determination by X-ray crystallography and solution NMR.

Endotoxin-tolerant mice have mutations in Toll-like receptor 4 (Tlr4)

Bacterial lipopolysaccharide (LPS) provokes a vigorous, generalized proinflammatory state in the infected host. Genetic regulation of this response has been localized to the Lps locus on mouse chromosome 4, through study of the C3H/HeJ and C57BL/10ScCr inbred strains. Both C3H/HeJ and C57BL/10ScCr mice are homozygous for a mutant Lps allele (Lpsd/d) that confers hyporesponsiveness to LPS challenge, and therefore exhibit natural tolerance to its lethal effects. Genetic and physical mapping of 1,345 backcross progeny segregating this mutant phenotype confined Lps to a 0.9-cM interval spanning 1.7 Mb. Three transcription units were identified within the candidate interval, including Toll-like receptor 4 (Tlr4), part of a protein family with members that have been implicated in LPS-induced cell signaling. C3H/HeJ mice have a point mutation within the coding region of the Tlr4 gene, resulting in a nonconservative substitution of a highly conserved proline by histidine at codon 712, whereas C57BL/ 10ScCr mice exhibit a deletion of Tlr4. Identification of distinct mutations involving the same gene at the Lps locus in two different hyporesponsive inbred mouse strains strongly supports the hypothesis that altered Tlr4 function is responsible for endotoxin tolerance.

Mammalian multidrug resistance gene: complete cDNA sequence indicates strong homology to bacterial transport proteins

The complete nucleotide and primary structure (1276 amino acids) of a full length mdr cDNA capable of conferring a complete multidrug-resistant phenotype is presented. The deduced amino acid sequence suggests that mdr is a membrane glycoprotein which includes six pairs of transmembrane domains and a cluster of potentially N-linked glycosylation sites near the amino terminus. A striking feature of the protein is an internal duplication that includes approximately 500 amino acids. Each duplicated segment includes a consensus ATP-binding site. Amino acid homology is observed between the mdr gene and a series of bacterial transport genes. This strong homology suggests that a highly conserved functional unit involved in membrane transport is present in the mdr polypeptide. We propose that an energy-dependent transport mechanism is responsible for the multidrug-resistant phenotype.

Natural resistance to infection with intracellular parasites: isolation of a candidate for Bcg

Natural resistance to infection with intracellular parasites is controlled by a dominant gene on mouse chromosome 1, called Bcg, Lsh, or Ity. Bcg affects the capacity of macrophages to destroy ingested intracellular parasites early during infection. We have assembled a 400 kb bacteriophage and cosmid contig within the genomic interval containing Bcg. A search for transcription units by exon amplification identified six novel genes in this contig. RNA expression studies showed that one of them, designated Nramp, was expressed exclusively in macrophage populations from reticuloendothelial organs and in the macrophage line J774A. Nramp encodes an integral membrane protein that has structural homology with known prokaryotic and eukaryotic transport systems, suggesting a macrophage-specific membrane transport function. Susceptibility to infection (Bcgs) in 13 Bcgr and Bcgs strains tested is associated with a nonconservative Gly-105 to Asp-105 substitution within predicted transmembrane domain 2 of Nramp.

Isolation and expression of a complementary DNA that confers multidrug resistance

The emergence and outgrowth of a population of tumour cells resistant to multiple drugs is a major problem in the chemotherapeutic treatment of cancer. We have used highly drug-resistant cell lines developed in vitro to study the molecular basis of multidrug resistance. In these cell lines high levels of resistance are frequently associated with amplification and overexpression of a small group of genes termed mdr or gp170. Direct evaluation of the role of these genes in multidrug resistance has awaited the isolation of a member of this gene family in a biologically active form. Here we report the isolation of DNA clones complementary to the cellular messenger RNA transcripts of mdr genes and show that high-level expression of a full-length complementary DNA clone in an otherwise drug-sensitive cell confers a complete multidrug-resistant phenotype. Our results demonstrate that overexpression of a single member of the mdr group is sufficient to confer drug resistance. Furthermore, because the cDNA was isolated from a drug-sensitive cell, mutations in the primary sequence of mdr are not required to produce a multidrug-resistance phenotype.

Complement is activated by IgG hexamers assembled at the cell surface

Complement activation by antibodies bound to pathogens, tumors, and self antigens is a critical feature of natural immune defense, a number of disease processes, and immunotherapies. How antibodies activate the complement cascade, however, is poorly understood. We found that specific noncovalent interactions between Fc segments of immunoglobulin G (IgG) antibodies resulted in the formation of ordered antibody hexamers after antigen binding on cells. These hexamers recruited and activated C1, the first component of complement, thereby triggering the complement cascade. The interactions between neighboring Fc segments could be manipulated to block, reconstitute, and enhance complement activation and killing of target cells, using all four human IgG subclasses. We offer a general model for understanding antibody-mediated complement activation and the design of antibody therapeutics with enhanced efficacy.

The R-spondin/Lgr5/Rnf43 module: regulator of Wnt signal strength

Adult stem cells are controlled by an intricate interplay of potent Wnt agonists, antagonists, and anti-antagonists. This review by de Lau et al. focuses on the complex physical and functional interactions of three recently discovered protein families that control stem cell activity by regulating surface expression of Wnt receptors: Lgr5 and its homologs, the E3 ligases Rnf43 and Znrf3, and the secreted R-spondin ligands.

Lgr5 was originally discovered as a common Wnt target gene in adult intestinal crypts and colon cancer. It was subsequently identified as an exquisite marker of multiple Wnt-driven adult stem cell types. Lgr5 and its homologs, Lgr4 and Lgr6, constitute the receptors for R-spondins, potent Wnt signal enhancers and stem cell growth factors. The Lgr5/R-spondin complex acts by neutralizing Rnf43 and Znrf3, two transmembrane E3 ligases that remove Wnt receptors from the stem cell surface. Rnf43/Znrf3 are themselves encoded by Wnt target genes and constitute a negative Wnt feedback loop. Thus, adult stem cells are controlled by an intricate interplay of potent Wnt agonists, antagonists, and anti-antagonists.

Splotch (Sp2H), a mutation affecting development of the mouse neural tube, shows a deletion within the paired homeodomain of Pax-3

The molecular basis of the mouse mutation splotch (Sp), which is associated with spina bifida and exencephaly, was analyzed at three of its alleles, Sp, Sp2H, and Spr. We mapped the paired box gene Pax-3 within the Inha to Akp3 interval, near or at the Sp locus on chromosome 1, and found Pax-3 to be deleted in heterozygous Spr/+ mice. Analysis of genomic DNA and cDNA clones constructed from Sp2H/Sp2H embryos identified a deletion of 32 nucleotides in the Pax-3 mRNA transcript and gene. This deletion maps within the paired homeodomain of PAX-3 and is predicted to create a truncated protein as a result of a newly created termination codon at the deletion breakpoint. Our study provides evidence for a causal link between deletion of the paired homeodomain of Pax-3 and the Sp2H mutation, and infers that Pax-3 plays a key role in normal neural development.

Structures of glycoprotein Ibalpha and its complex with von Willebrand factor A1 domain

Transient interactions of platelet-receptor glycoprotein Ibalpha (GpIbalpha) and the plasma protein von Willebrand factor (VWF) reduce platelet velocity at sites of vascular damage and play a role in haemostasis and thrombosis. Here we present structures of the GpIbalpha amino-terminal domain and its complex with the VWF domain A1. In the complex, GpIbalpha wraps around one side of A1, providing two contact areas bridged by an area of solvated charge interaction. The structures explain the effects of gain-of-function mutations related to bleeding disorders and provide a model for shear-induced activation. These detailed insights into the initial interactions in platelet adhesion are relevant to the development of antithrombotic drugs.

Phosphatidylcholine translocase: a physiological role for the mdr2 gene

P-glycoproteins (P-gps) encoded by the mouse mdr2 and mdr3 genes were expressed in secretory vesicles (SVs) from the yeast mutant sec6-4, and their capacity to function as a lipid translocase/flippase was tested. An assay that uses a fluorescent phosphatidylcholine (PC) analog was developed to quantitate asymmetric lipid distribution in the outer and inner leaflets of the lipid bilayer of these vesicles. Mdr2 expression in SVs caused a time- and temperature-dependent enhancement of PC translocation to the inner leaflet of the membrane. The Mdr2-mediated effect was specific since expression of Mdr3 in these vesicles was without effect on the membrane distribution of PC. Increased Mdr2-mediated PC translocation was strictly ATP and Mg2+ dependent, was abrogated by the ATPase inhibitor vanadate and the P-gp modulator verapamil, but was insensitive to the presence of excess of the multidrug resistance drugs colchicine and vinblastine.

The Ity/Lsh/Bcg locus: natural resistance to infection with intracellular parasites is abrogated by disruption of the Nramp1 gene

In mice, natural resistance or susceptibility to infection with intracellular parasites is determined by a locus or group of loci on chromosome 1, designated Bcg, Lsh, and Ity, which controls early microbial replication in reticuloendothelial organs. We have identified by positional cloning a candidate gene for Bcg, Nramp1, which codes for a novel macrophage-specific membrane transport protein. We have created a mouse mutant bearing a null allele at Nramp1, and we have analyzed the effect of such a mutation on natural resistance to infection. Targeted disruption of Nramp1 has pleiotropic effects on natural resistance to infection with intracellular parasites, as it eliminated resistance to Mycobacterium bovis, Leishmania donovani, and lethal Salmonella typhimurium infection, establishing that Nramp1, Bcg, Lsh, and Ity are the same locus. Comparing the profiles of parasite replication in control and Nramp1-/- mice indicated that the Nramp1Asp169 allele of BcgS inbred strains is a null allele, pointing to a critical role of this residue in the mechanism of action of the protein. Despite their inability to control parasite growth in the early nonimmune phase of the infection, Nramp1-/- mutants can overcome the infection in the late immune phase, suggesting that Nramp1 plays a key role only in the early part of the macrophage-parasite interaction and may function by a cytocidal or cytostatic mechanism distinct from those expressed by activated macrophages.

Isolation of human mdr DNA sequences amplified in multidrug-resistant KB carcinoma cells

The ability of tumor cells to develop simultaneous resistance to structurally different cytotoxic drugs constitutes a major problem in cancer chemotherapy. It was previously demonstrated that multidrug-resistant Chinese hamster cell lines contain an amplified, transcriptionally active DNA sequence designated mdr. This report presents evidence that multidrug-resistant sublines of human KB carcinoma cells, selected for resistance to either colchicine, vinblastine, or Adriamycin (doxorubicin), display amplification of two different DNA sequences homologous to the hamster mdr gene. Segments of the human mdr DNA sequences, designated mdr1 and mdr2, have been cloned. mdr1 sequences were amplified in all of the highly drug-resistant sublines and were expressed as a poly(A)+ RNA species of 4.5 kilobases that was detected in the resistant cells but not in the parental cell line. No expression of mdr2 sequences was detected. mdr2 sequences were coamplified with mdr1 in some of the multidrug-resistant sublines and, in two independently derived cell lines, underwent very similar rearrangements. The data suggest that the mdr1 gene is involved in multidrug resistance in human cells.

Structures of complement component C3 provide insights into the function and evolution of immunity

The mammalian complement system is a phylogenetically ancient cascade system that has a major role in innate and adaptive immunity. Activation of component C3 (1,641 residues) is central to the three complement pathways and results in inflammation and elimination of self and non-self targets. Here we present crystal structures of native C3 and its final major proteolytic fragment C3c. The structures reveal thirteen domains, nine of which were unpredicted, and suggest that the proteins of the alpha2-macroglobulin family evolved from a core of eight homologous domains. A double mechanism prevents hydrolysis of the thioester group, essential for covalent attachment of activated C3 to target surfaces. Marked conformational changes in the alpha-chain, including movement of a critical interaction site through a ring formed by the domains of the beta-chain, indicate an unprecedented, conformation-dependent mechanism of activation, regulation and biological function of C3.

Ltap, a mammalian homolog of Drosophila Strabismus/Van Gogh, is altered in the mouse neural tube mutant Loop-tail

Neural tube defects (NTDs) such as spina bifida and anencephaly are common congenital malformations in humans (1/1,000 births) that result from failure of the neural tube to close during embryogenesis. The etiology of NTDs is complex, with both genetic and environmental contributions; the genetic component has been extensively studied with mouse models. Loop-tail (Lp) is a semidominant mutation on mouse chromosome 1 (ref. 4). In the two known Lp alleles (Lp, Lpm1Jus), heterozygous mice exhibit a characteristic looped tail, and homozygous embryos show a completely open neural tube in the hindbrain and spinal region, a condition similar to the severe craniorachischisis defect in humans. Morphological and neural patterning studies indicate a role for the Lp gene product in controlling early morphogenesis and patterning of both axial midline structures and the developing neural plate. The 0.6-cM/0.7-megabase (Mb) Lp interval is delineated proximally by D1Mit113/Apoa2/Fcer1g and distally by Fcer1a/D1Mit149/Spna1 and contains a minimum of 17 transcription units. One of these genes, Ltap, encodes a homolog of Drosophila Strabismus/Van Gogh (Stbm/Vang), a component of the frizzled/dishevelled tissue polarity pathway. Ltap is expressed broadly in the neuroectoderm throughout early neurogenesis and is altered in two independent Lp alleles, identifying this gene as a strong candidate for Lp.

Specific chemical and structural damage to proteins produced by synchrotron radiation

Radiation damage is an inherent problem in x-ray crystallography. It usually is presumed to be nonspecific and manifested as a gradual decay in the overall quality of data obtained for a given crystal as data collection proceeds. Based on third-generation synchrotron x-ray data, collected at cryogenic temperatures, we show for the enzymes Torpedo californica acetylcholinesterase and hen egg white lysozyme that synchrotron radiation also can cause highly specific damage. Disulfide bridges break, and carboxyl groups of acidic residues lose their definition. Highly exposed carboxyls, and those in the active site of both enzymes, appear particularly susceptible. The catalytic triad residue, His-440, in acetylcholinesterase, also appears to be much more sensitive to radiation damage than other histidine residues. Our findings have direct practical implications for routine x-ray data collection at high-energy synchrotron sources. Furthermore, they provide a direct approach for studying the radiation chemistry of proteins and nucleic acids at a detailed, structural level and also may yield information concerning putative "weak links" in a given biological macromolecule, which may be of structural and functional significance.

Natural resistance to infection with intracellular pathogens: the Nramp1 protein is recruited to the membrane of the phagosome

The Nramp1 (natural-resistance-associated macrophage protein 1) locus (Bcg, Ity, Lsh) controls the innate resistance or susceptibility of mice to infection with a group of unrelated intracellular parasites which includes Salmonella, Leishmania, and Mycobacterium. Nramp1 is expressed exclusively in professional phagocytes and encodes an integral membrane protein that shares structural characteristics with ion channels and transporters. Its function and mechanism of action remain unknown. The intracellular localization of the Nramp1 protein was analyzed in control 129/sv and mutant Nramp1-/- macrophages by immunofluorescence and confocal microscopy and by biochemical fractionation. In colocalization studies with a specific anti-Nramp1 antiserum and a panel of control antibodies directed against known cellular structures, Nramp1 was found not to be expressed at the plasma membrane but rather localized to the late endocytic compartments (late endosome/lysosome) of resting macrophages in a Lamp1 (lysosomal-associated membrane protein 1)-positive compartment. Double immunofluorescence studies and direct purification of latex bead-containing phagosomes demonstrated that upon phagocytosis, Nramp1 is recruited to the membrane of the phagosome and remains associated with this structure during its maturation to phagolysosome. After phagocytosis, Nramp1 is acquired by the phagosomal membrane with time kinetics similar to Lamp1, but clearly distinct from those of the early endosomal marker Rab5. The targeting of Nramp1 from endocytic vesicles to the phagosomal membrane supports the hypothesis that Nramp1 controls the replication of intracellular parasites by altering the intravacuolar environment of the microbe-containing phagosome.

Divalent-metal transport by NRAMP proteins at the interface of host-pathogen interactions

The NRAMP family of divalent-metal transporters plays a key role in the homeostasis of iron and other metals. NRAMP2 (DMT1) acts as an iron-uptake protein in both the duodenum and in peripheral tissues. NRAMP1 functions as a divalent-metal efflux pump at the phagosomal membrane of macrophages and neutrophils, and mutations in NRAMP1 cause susceptibility to several intracellular pathogens. NRAMP homologues have been identified in bacteria and are involved in acquiring divalent metals from the extracellular environment. Interestingly, bacterial and mammalian NRAMP proteins would compete for the same essential substrates within the microenvironment of the phagosome, at the interface of host-pathogen interactions.

Natural resistance to intracellular infections: natural resistance-associated macrophage protein 1 (Nramp1) functions as a pH-dependent manganese transporter at the phagosomal membrane

Mutations at the natural resistance-associated macrophage protein 1 (Nramp1) locus cause susceptibility to infection with antigenically unrelated intracellular pathogens. Nramp1 codes for an integral membrane protein expressed in the lysosomal compartment of macrophages, and is recruited to the membrane of phagosomes soon after the completion of phagocytosis. To define whether Nramp1 functions as a transporter at the phagosomal membrane, a divalent cation-sensitive fluorescent probe was designed and covalently attached to a porous particle. The resulting conjugate, zymosan-FF6, was ingested by macrophages and its fluorescence emission was recorded in situ after phagocytosis, using digital imaging. Quenching of the probe by Mn(2+) was used to monitor the flux of divalent cations across the phagosomal membrane in peritoneal macrophages obtained from Nramp1-expressing (+/+) and Nramp1-deficient (-/-) macrophages. Phagosomes from Nramp1(+/+) mice extrude Mn(2+) faster than their Nramp(-/-) counterparts. The difference in the rate of transport is eliminated when acidification of the phagosomal lumen is dissipated, suggesting that divalent metal transport through Nramp1 is H(+) dependent. These studies suggest that Nramp1 contributes to defense against infection by extrusion of divalent cations from the phagosomal space. Such cations are likely essential for microbial function and their removal from the phagosomal microenvironment impairs pathogenesis, resulting in enhanced bacteriostasis or bactericidal activity.

Structure of the translocator domain of a bacterial autotransporter

Autotransporters are virulence-related proteins of Gram-negative bacteria that are secreted via an outer-membrane-based C-terminal extension, the translocator domain. This domain supposedly is sufficient for the transport of the N-terminal passenger domain across the outer membrane. We present here the crystal structure of the in vitro-folded translocator domain of the autotransporter NalP from Neisseria meningitidis, which reveals a 12-stranded beta-barrel with a hydrophilic pore of 10 x 12.5 A that is filled by an N-terminal alpha-helix. The domain has pore activity in vivo and in vitro. Our data are consistent with the model of passenger-domain transport through the hydrophilic channel within the beta-barrel, and inconsistent with a model for transport through a central channel formed by an oligomer of translocator domains. However, the dimensions of the pore imply translocation of the secreted domain in an unfolded form. An alternative model, possibly covering the transport of folded domains, is that passenger-domain transport involves the Omp85 complex, the machinery required for membrane insertion of outer-membrane proteins, on which autotransporters are dependent.

Structure of C3b reveals conformational changes that underlie complement activity

Resistance to infection and clearance of cell debris in mammals depend on the activation of the complement system, which is an important component of innate and adaptive immunity. Central to the complement system is the activated form of C3, called C3b, which attaches covalently to target surfaces to amplify complement response, label cells for phagocytosis and stimulate the adaptive immune response. C3b consists of 1,560 amino-acid residues and has 12 domains. It binds various proteins and receptors to effect its functions. However, it is not known how C3 changes its conformation into C3b and thereby exposes its many binding sites. Here we present the crystal structure at 4-A resolution of the activated complement protein C3b and describe the conformational rearrangements of the 12 domains that take place upon proteolytic activation. In the activated form the thioester is fully exposed for covalent attachment to target surfaces and is more than 85 A away from the buried site in native C3 (ref. 5). Marked domain rearrangements in the alpha-chain present an altered molecular surface, exposing hidden and cryptic sites that are consistent with known putative binding sites of factor B and several complement regulators. The structural data indicate that the large conformational changes in the proteolytic activation and regulation of C3 take place mainly in the first conversion step, from C3 to C3b. These insights are important for the development of strategies to treat immune disorders that involve complement-mediated inflammation.

Complement component C3 - The "Swiss Army Knife" of innate immunity and host defense

As a preformed defense system, complement faces a delicate challenge in providing an immediate, forceful response to pathogens even at first encounter, while sparing host cells in the process. For this purpose, it engages a tightly regulated network of plasma proteins, cell surface receptors, and regulators. Complement component C3 plays a particularly versatile role in this process by keeping the cascade alert, acting as a point of convergence of activation pathways, fueling the amplification of the complement response, exerting direct effector functions, and helping to coordinate downstream immune responses. In recent years, it has become evident that nature engages the power of C3 not only to clear pathogens but also for a variety of homeostatic processes ranging from tissue regeneration and synapse pruning to clearing debris and controlling tumor cell progression. At the same time, its central position in immune surveillance makes C3 a target for microbial immune evasion and, if improperly engaged, a trigger point for various clinical conditions. In our review, we look at the versatile roles and evolutionary journey of C3, discuss new insights into the molecular basis for C3 function, provide examples of disease involvement, and summarize the emerging potential of C3 as a therapeutic target.

Structure of complement fragment C3b-factor H and implications for host protection by complement regulators

Factor H (FH) is an abundant regulator of complement activation and protects host cells from self-attack by complement. Here we provide insight into the regulatory activity of FH by solving the crystal structure of the first four domains of FH in complex with its target, complement fragment C3b. FH interacted with multiple domains of C3b, covering a large, extended surface area. The structure indicated that FH destabilizes the C3 convertase by competition and electrostatic repulsion and that FH enables proteolytic degradation of C3b by providing a binding platform for protease factor I while stabilizing the overall domain arrangement of C3b. Our results offer general models for complement regulation and provide structural explanations for disease-related mutations in the genes encoding both FH and C3b.

Adhesion mechanism of human beta(2)-glycoprotein I to phospholipids based on its crystal structure

Human beta(2)-glycoprotein I is a heavily glycosylated five-domain plasma membrane-adhesion protein, which has been implicated in blood coagulation and clearance of apoptotic bodies from the circulation. It is also the key antigen in the autoimmune disease anti-phospholipid syndrome. The crystal structure of beta(2)-glycoprotein I isolated from human plasma reveals an elongated fish-hook-like arrangement of the globular short consensus repeat domains. Half of the C-terminal fifth domain deviates strongly from the standard fold, as observed in domains one to four. This aberrant half forms a specific phospholipid-binding site. A large patch of 14 positively charged residues provides electrostatic interactions with anionic phospholipid headgroups and an exposed membrane-insertion loop yields specificity for lipid layers. The observed spatial arrangement of the five domains suggests a functional partitioning of protein adhesion and membrane adhesion over the N- and C-terminal domains, respectively, separated by glycosylated bridging domains. Coordinates are in the Protein Data Bank (accession No. 1QUB).

The iron transport protein NRAMP2 is an integral membrane glycoprotein that colocalizes with transferrin in recycling endosomes

The natural resistance associated macrophage protein (Nramp) gene family is composed of two members in mammals, Nramp1 and Nramp2. Nramp1 is expressed primarily in macrophages and mutations at this locus cause susceptibility to infectious diseases. Nramp2 has a much broader range of tissue expression and mutations at Nramp2 result in iron deficiency, indicating a role for Nramp2 in iron metabolism. To get further insight into the function and mechanism of action of Nramp proteins, we have generated isoform specific anti-Nramp1 and anti-Nramp2 antisera. Immunoblotting experiments indicate that Nramp2 is present in a number of cell types, including hemopoietic precursors, and is coexpressed with Nramp1 in primary macrophages and macrophage cell lines. Nramp2 is expressed as a 90-100-kD integral membrane protein extensively modified by glycosylation (>40% of molecular mass). Subcellular localization studies by immunofluorescence and confocal microscopy indicate distinct and nonoverlapping localization for Nramp1 and Nramp2. Nramp1 is expressed in the lysosomal compartment, whereas Nramp2 is not detectable in the lysosomes but is expressed primarily in recycling endosomes and also, to a lower extent, at the plasma membrane, colocalizing with transferrin. These findings suggest that Nramp2 plays a key role in the metabolism of transferrin-bound iron by transporting free Fe2+ across the endosomal membrane and into the cytoplasm.