Single particle reconstruction of the human apo-transferrin-transferrin receptor complex

Transferrin receptor binds virus capsid with dynamic motion

Canine parvovirus (CPV) is an important pathogen causing severe diseases in dogs, including acute hemorrhagic enteritis, myocarditis, and cerebellar disease. Cross-species transmission of CPV occurs as a result of mutations on the viral capsid surface that alter the species-specific binding to the host receptor, transferrin receptor type-1 (TfR). The interaction between CPV and TfR has been extensively studied, and previous analyses have suggested that the CPV-TfR complex is asymmetric. To enhance the understanding of the underlying molecular mechanisms, we determined the CPV-TfR interaction using cryo-electron microscopy to solve the icosahedral (3.0-Å resolution) and asymmetric (5.0-Å resolution) complex structures. Structural analyses revealed conformational variations of the TfR molecules relative to the binding site, which translated into dynamic molecular interactions between CPV and TfR. The precise footprint of the receptor on the virus capsid was identified, along with the identity of the amino acid residues in the virus-receptor interface. Our "rock-and-roll" model provides an explanation for previous findings and gives insights into species jumping and the variation in host ranges associated with new pandemics in dogs.

Cryo-EM structure of the human ferritin-transferrin receptor 1 complex

Human transferrin receptor 1 (CD71) guarantees iron supply by endocytosis upon binding of iron-loaded transferrin and ferritin. Arenaviruses and the malaria parasite exploit CD71 for cell invasion and epitopes on CD71 for interaction with transferrin and pathogenic hosts were identified. Here, we provide the molecular basis of the CD71 ectodomain-human ferritin interaction by determining the 3.9 Å resolution single-particle cryo-electron microscopy structure of their complex and by validating our structural findings in a cellular context. The contact surfaces between the heavy-chain ferritin and CD71 largely overlap with arenaviruses and Plasmodium vivax binding regions in the apical part of the receptor ectodomain. Our data account for transferrin-independent binding of ferritin to CD71 and suggest that select pathogens may have adapted to enter cells by mimicking the ferritin access gate.

The human transferrin receptor 1 (CD71) is a transmembrane protein responsible for iron uptake. Here the authors present the 3.9 Å resolution cryo-EM structure of the CD71 ectodomain-human ferritin (H-Ft) complex and find that H-Ft binds a CD71 region different from the transferrin one that overlaps with the surface recognized by select pathogens.

Update on iron metabolism and molecular perspective of common genetic and acquired disorder, hemochromatosis

Iron is an essential component of erythropoiesis and its metabolism is tightly regulated by a variety of internal and external cues including iron storage, tissue hypoxia, inflammation and degree of erythropoiesis. There has been remarkable improvement in our understanding of the molecular mechanisms of iron metabolism past decades. The classical model of iron metabolism with iron response element/iron response protein (IRE/IRP) is now extended to include hepcidin model. Endogenous and exogenous signals funnel down to hepcidin via wide range of signaling pathways including Janus Kinase/Signal Transducer and Activator of Transcription 3 (JAK/STAT3), Bone Morphogenetic Protein/Hemojuvelin/Mothers Against Decapentaplegic Homolog (BMP/HJV/SMAD), and Von Hippel Lindau/Hypoxia-inducible factor/Erythropoietin (VHL/HIF/EPO), then relay to ferroportin, which directly regulates intra- and extracellular iron levels. The successful molecular delineation of iron metabolism further enhanced our understanding of common genetic and acquired disorder, hemochromatosis. The majority of the hereditary hemochromatosis (HH) patients are now shown to have mutations in the genes coding either upstream or downstream proteins of hepcidin, resulting in iron overload. The update on hepcidin centered mechanisms of iron metabolism and their clinical perspective in hemochromatosis will be discussed in this review.

Evolutionary reconstructions of the transferrin receptor of Caniforms supports canine parvovirus being a re-emerged and not a novel pathogen in dogs

Parvoviruses exploit transferrin receptor type-1 (TfR) for cellular entry in carnivores, and specific interactions are key to control of host range. We show that several key mutations acquired by TfR during the evolution of Caniforms (dogs and related species) modified the interactions with parvovirus capsids by reducing the level of binding. These data, along with signatures of positive selection in the TFRC gene, are consistent with an evolutionary arms race between the TfR of the Caniform clade and parvoviruses. As well as the modifications of amino acid sequence which modify binding, we found that a glycosylation site mutation in the TfR of dogs which provided resistance to the carnivore parvoviruses which were in circulation prior to about 1975 predates the speciation of coyotes and dogs. Because the closely-related black-backed jackal has a TfR similar to their common ancestor and lacks the glycosylation site, reconstructing this mutation into the jackal TfR shows the potency of that site in blocking binding and infection and explains the resistance of dogs until recent times. This alters our understanding of this well-known example of viral emergence by indicating that canine parvovirus emergence likely resulted from the re-adaptation of a parvovirus to the resistant receptor of a former host.

Parvoviruses in cats and dogs have been studied as a model system to understand how viruses gain the ability to infect new host species. By studying the evolution of the transferrin receptor, which the virus uses to enter a cell, we discovered that the ancestors of dogs were likely infected by a parvovirus millions of years ago until they evolved and became resistant; this was caused by their transferrin receptor changing so it no longer bound the virus. When a variant virus that infects dogs emerged in the 1970s, it had adapted to overcome this block. This story suggests that diseases which were once eliminated from a species can evolve and regain the infectivity for that host, therefore having high potential to be emerging diseases. We identified features of the receptor that were important to the evolution of this host-virus interaction and confirmed their role in regulating virus binding in cell culture.

Advances and challenges in analytical characterization of biotechnology products: mass spectrometry-based approaches to study properties and behavior of protein therapeutics

Biopharmaceuticals are a unique class of medicines due to their extreme structural complexity. The structure of these therapeutic proteins is critically important for their efficacy and safety, and the ability to characterize it at various levels (from sequence to conformation) is critical not only at the quality control stage, but also throughout the discovery and design stages. Biological mass spectrometry (MS) offers a variety of approaches to study structure and behavior of complex protein drugs and has already become a default tool for characterizing the covalent structure of protein therapeutics, including sequence and post-translational modifications. Recently, MS-based methods have also begun enjoying a dramatic growth in popularity as a means to provide information on higher order structure and dynamics of biotechnology products. In particular, hydrogen/deuterium exchange MS and charge state distribution analysis of protein ions in electrospray ionization (ESI) MS offer a convenient way to assess the integrity of protein conformation. Native ESI MS also allows the interactions of protein drugs with their therapeutic targets and other physiological partners to be monitored using simple model systems. MS-based methods are also applied to study pharmacokinetics of biopharmaceutical products, where they begin to rival traditional immunoassays. MS already provides valuable support to all stages of development of biopharmaceuticals, from discovery to post-approval monitoring, and its impact on the field of biopharmaceutical analysis will undoubtedly continue to grow.

Expression, purification, and characterization of recombinant human transferrin from rice (Oryza sativa L.)

Transferrin is an essential ingredient used in cell culture media due to its crucial role in regulating cellular iron uptake, transport, and utilization. It is also a promising drug carrier used to increase a drug's therapeutic index via the unique transferrin receptor-mediated endocytosis pathway. Due to the high risk of contamination with blood-borne pathogens from the use of human or animal plasma-derived transferrin, recombinant transferrin is preferred for use as a replacement for native transferrin. We expressed recombinant human transferrin in rice (Oryza sativa L.) at a high level of 1% seed dry weight (10 g/kg). The recombinant human transferrin was able to be extracted with saline buffers and then purified by a one step anion exchange chromatographic process to greater than 95% purity. The rice-derived recombinant human transferrin was shown to be not only structurally similar to the native human transferrin, but also functionally the same as native transferrin in terms of reversible iron binding and promoting cell growth and productivity. These results indicate that rice-derived recombinant human transferrin should be a safe and low cost alternative to human or animal plasma-derived transferrin for use in cell culture-based biopharmaceutical production of protein therapeutics and vaccines.

Multivalent display and receptor-mediated endocytosis of transferrin on virus-like particles

The structurally regular and stable self-assembled capsids derived from viruses can be used as scaffolds for the display of multiple copies of cell- and tissue-targeting molecules and therapeutic agents in a convenient and well-defined manner. The human iron-transfer protein transferrin, a high affinity ligand for receptors upregulated in a variety of cancers, has been arrayed on the exterior surface of the protein capsid of bacteriophage Qbeta. Selective oxidation of the sialic acid residues on the glycan chains of transferrin was followed by introduction of a terminal alkyne functionality through an oxime linkage. Attachment of the protein to azide-functionalized Qbeta capsid particles in an orientation allowing access to the receptor binding site was accomplished by the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction. Transferrin conjugation to Qbeta particles allowed specific recognition by transferrin receptors and cellular internalization through clathrin-mediated endocytosis, as determined by fluorescence microscopy on cells expressing GFP-labeled clathrin light chains. By testing Qbeta particles bearing different numbers of transferrin molecules, it was demonstrated that cellular uptake was proportional to ligand density, but that internalization was inhibited by equivalent concentrations of free transferrin. These results suggest that cell targeting with transferrin can be improved by local concentration (avidity) effects.

Noncanonical interactions between serum transferrin and transferrin receptor evaluated with electrospray ionization mass spectrometry

The primary route of iron acquisition in vertebrates is the transferrin receptor (TfR) mediated endocytotic pathway, which provides cellular entry to the metal transporter serum transferrin (Tf). Despite extensive research efforts, complete understanding of Tf-TfR interaction mechanism is still lacking owing to the complexity of this system. Electrospray ionization mass spectrometry (ESI MS) is used in this study to monitor the protein/receptor interaction and demonstrate the ability of metal-free Tf to associate with TfR at neutral pH. A set of Tf variants is used in a series of competition and displacement experiments to bracket TfR affinity of apo-Tf at neutral pH (0.2-0.6 microM). Consistent with current models of endosomal iron release from Tf, acidification of the protein solution results in a dramatic change of binding preferences, with apo-Tf becoming a preferred receptor binder. Contrary to the current models implying that the apo-Tf/TfR complex dissociates almost immediately upon exposure to the neutral environment at the cell surface, our data indicate that this complex remains intact. Iron-loaded Tf displaces apo-Tf from TfR, making it available for the next cycle of iron binding, transport and delivery to tissues. However, apo-Tf may still interfere with the cellular uptake of engineered Tf molecules whose TfR affinity is affected by various modifications (e.g., conjugation to cytotoxic molecules). This work also highlights the great potential of ESI MS as a tool capable of providing precise details of complex protein-receptor interactions under conditions that closely mimic the environment in which these encounters occur in physiological systems.

High-resolution structural analysis of a DNA nanostructure by cryoEM

Many DNA nanostructures have been produced and a wide range of potential applications have been proposed. However, confirmation of accurate 3D construction is particularly challenging. Here, we demonstrate that cryoEM may be exploited to obtain structural information at sufficient resolution to visualize the DNA helix and reveal the absolute stereochemistry of a 7 nm self-assembled DNA tetrahedron. Structural analysis at such high resolution by cryoEM image analysis is unprecedented for any biological molecule of this size.

A structural comparison of human serum transferrin and human lactoferrin

The transferrins are a family of proteins that bind free iron in the blood and bodily fluids. Serum transferrins function to deliver iron to cells via a receptor-mediated endocytotic process as well as to remove toxic free iron from the blood and to provide an anti-bacterial, low-iron environment. Lactoferrins (found in bodily secretions such as milk) are only known to have an anti-bacterial function, via their ability to tightly bind free iron even at low pH, and have no known transport function. Though these proteins keep the level of free iron low, pathogenic bacteria are able to thrive by obtaining iron from their host via expression of outer membrane proteins that can bind to and remove iron from host proteins, including both serum transferrin and lactoferrin. Furthermore, even though human serum transferrin and lactoferrin are quite similar in sequence and structure, and coordinate iron in the same manner, they differ in their affinities for iron as well as their receptor binding properties: the human transferrin receptor only binds serum transferrin, and two distinct bacterial transport systems are used to capture iron from serum transferrin and lactoferrin. Comparison of the recently solved crystal structure of iron-free human serum transferrin to that of human lactoferrin provides insight into these differences.

The crystal structure of iron-free human serum transferrin provides insight into inter-lobe communication and receptor binding

Serum transferrin reversibly binds iron in each of two lobes and delivers it to cells by a receptor-mediated, pH-dependent process. The binding and release of iron result in a large conformational change in which two subdomains in each lobe close or open with a rigid twisting motion around a hinge. We report the structure of human serum transferrin (hTF) lacking iron (apo-hTF), which was independently determined by two methods: 1) the crystal structure of recombinant non-glycosylated apo-hTF was solved at 2.7-A resolution using a multiple wavelength anomalous dispersion phasing strategy, by substituting the nine methionines in hTF with selenomethionine and 2) the structure of glycosylated apo-hTF (isolated from serum) was determined to a resolution of 2.7A by molecular replacement using the human apo-N-lobe and the rabbit holo-C1-subdomain as search models. These two crystal structures are essentially identical. They represent the first published model for full-length human transferrin and reveal that, in contrast to family members (human lactoferrin and hen ovotransferrin), both lobes are almost equally open: 59.4 degrees and 49.5 degrees rotations are required to open the N- and C-lobes, respectively (compared with closed pig TF). Availability of this structure is critical to a complete understanding of the metal binding properties of each lobe of hTF; the apo-hTF structure suggests that differences in the hinge regions of the N- and C-lobes may influence the rates of iron binding and release. In addition, we evaluate potential interactions between apo-hTF and the human transferrin receptor.