Human serum transferrin (hTF) binds Fe(III) tightly but reversibly, and delivers it to cells via a receptor-mediated endocytosis process. The metal-binding and release result in significant conformational changes of the protein. Here, we report the crystal structures of diferric-hTF (FeNFeC-hTF) and bismuth-bound hTF (BiNFeC-hTF) at 2.8 and 2.4 Å resolutions respectively. Notably, the N-lobes of both structures exhibit unique “partially-opened” conformations between those of the apo-hTF and holo-hTF. Fe(III) and Bi(III) in the N-lobe coordinate to, besides anions, only two (Tyr95 and Tyr188) and one (Tyr188) tyrosine residues, respectively, in contrast to four residues in the holo-hTF. The C-lobe of both structures are fully closed with iron coordinating to four residues and a carbonate. The structures of hTF observed here represent key conformers captured in the dynamic nature of the transferrin family proteins and provide a structural basis for understanding the mechanism of metal uptake and release in transferrin families.
The transferrins are a family of bilobal iron-binding proteins that play the crucial role of binding ferric iron and keeping it in solution, thereby controlling the levels of this important metal. Human serum transferrin (hTF) carries one iron in each of two similar lobes. Understanding the detailed mechanism of iron release from each lobe of hTF during receptor mediated endocytosis has been extremely challenging because of the active participation of the transferrin receptor (TFR), salt, a chelator, lobe-lobe interactions and the low pH within the endosome. Our use of authentic monoferric hTF (unable to bind iron in one lobe) or of diferric hTF (with iron locked in one lobe), provided distinct kinetic end points allowing us to bypass many of the previous difficulties. The capture and unambiguous assignment of all kinetic events associated with iron release by stopped flow spectrofluorimetry, in the presence and absence of the TFR, unequivocally establishes the decisive role of the TFR in promoting efficient and balanced iron release from both lobes of hTF during one endocytic cycle. For the first time the four microscopic rate constants required to accurately describe the kinetics of iron removal are reported for hTF with and without the TFR. Specifically, at pH 5.6, the TFR enhances the rate of iron release from the C-lobe (7- to 11-fold), and slows the rate of iron release from the N-lobe (6- to 15-fold), making them more equivalent and producing an increase in the net rate of iron removal from Fe2hTF. Calculated cooperativity factors, in addition to plots of time dependent species distributions in the absence and presence of the TFR clearly illustrate the differences. Accurate rate constants for the pH and salt induced conformational changes in each lobe precisely delineate how delivery of iron within the physiologically relevant time frame of 2 min might be accomplished.
Stopped flow fluorescence; iron release kinetics; salt effect; iron release model
Human serum transferrin (hTF) is a bilobal glycoprotein that reversibly binds Fe3+ and delivers it to cells by the process of receptor-mediated endocytosis. Despite decades of research, the precise events resulting in iron release from each lobe of hTF within the endosome have not been fully delineated.
Scope of Review
We provide an overview of the kinetics of iron release from hTF ± the transferrin receptor (TFR) at endosomal pH (5.6). A critical evaluation of the array of biophysical techniques used to determine accurate rate constants is provided.
Delivery of Fe3+ by to actively dividing cells by hTF is essential; too much or too little Fe3+ directly impacts the well-being of an individual. Because the interaction of hTF with the TFR controls iron distribution in the body, an understanding of this process at the molecular level is essential.
Not only does TFR direct the delivery of iron to the cell through the binding of hTF, kinetic data demonstrate that it also modulates iron release from the N- and C-lobes of hTF. Specifically, the TFR balances the rate of iron release from each lobe, resulting in efficient Fe3+ release within a physiologically relevant time frame.
transferrin; transferrin receptor; kinetics; fluorescence
Essential to iron transport and delivery, human serum transferrin (hTF) is a bilobal glycoprotein capable of reversibly binding one ferric ion in each lobe (the N- and C-lobes). A complete description of iron release from hTF, as well as insight into the physiological significance of the bilobal structure, demands characterization of the isolated lobes. Although production of large amounts of isolated N-lobe and full-length hTF has been well documented, attempts to produce the C-lobe (by recombinant and/or proteolytic approaches) have met with more limited success. Our new strategy involves replacing the hepta-peptide, PEAPTDE (comprising the bridge between the lobes) with the sequence ENLYFQ/G in a His-tagged non-glycosylated monoferric hTF construct, designated FeChTF. The new bridge sequence of this construct, designated FeCTEV hTF, is readily cleaved by the tobacco etch virus (TEV) protease yielding non-glycosylated C-lobe. Following nickel column chromatography (to remove the N-lobe and the TEV protease which are both His tagged), the homogeneity of the C-lobe has been confirmed by mass spectroscopy. Differing reactivity with a monoclonal antibody specific to the C-lobe indicates that introduction of the TEV cleavage site into the bridge alters its conformation. The spectral and kinetic properties of the isolated C-lobe differ significantly from those of the isolated N-lobe.
Tobacco etch virus protease; transferrin; stopped-flow fluorescence; intrinsic tryptophan fluorescence; absorption coefficient determination; cooperativity
Human serum transferrin (hTF), with two Fe3+ binding lobes transports iron into cells. Diferric hTF preferentially binds to a specific receptor (TFR) on the surface of cells and the complex undergoes clathrin dependent receptor-mediated endocytosis. The clathrin-coated vesicle fuses with an endosome where the pH is lowered, facilitating iron release from hTF. On a biologically relevant timescale (2-3 min), the factors critical to iron release include pH, anions, a chelator and the interaction of hTF with the TFR. Previous work, in which the increase in the intrinsic fluorescence signal was used to monitor iron release from the hTF/TFR complex, established that the TFR significantly enhances the rate of iron release from the C-lobe of hTF. In the current study, the role of the five C-lobe Trp residues in reporting the fluorescence change has been evaluated (± sTFR). Only four of the five recombinant Trp→ Phe mutants produced well. A single slow rate constant for iron release is found for the monoferric C-lobe (FeC hTF) and the four Trp mutants in the FeC hTF background. The three Trp residues equivalent to those in the N-lobe differed from the N-lobe and each other in their contributions to the fluorescent signal. Two rate constants are observed for the FeC hTF control and the four Trp mutants in complex with the TFR: kobsC1 reports conformational change(s) in the C-lobe initiated by the TFR and kobsC2 is ascribed to iron release. Excitation at 295 nm (Trp only) and at 280 nm (Trp and Tyr) reveals interesting and significant differences in the rate constants for the complex.
Human serum transferrin (hTF) binds two ferric iron ions which are delivered to cells in a transferrin receptor (TFR) mediated process. Critical to the delivery of iron to cells is the binding of hTF to the TFR and the efficient release of iron orchestrated by the interaction. Within the endosome, iron release from hTF is also aided by lower pH, the presence of anions, and a chelator yet to be identified. We have recently shown that three of the four residues comprising a loop in the N-lobe (Pro142, Lys144, and Pro145) are critical to the high-affinity interaction of hTF with the TFR. In contrast, Arg143 in this loop does not participate in the binding isotherm. In the current study, the kinetics of iron release from alanine mutants of each of these four residues (placed into both diferric and monoferric N-lobe backgrounds) have been determined ± the TFR. The R143A mutation greatly retards the rate of iron release from the N-lobe in the absence of the TFR but has considerably less of an effect in its presence. Our data definitively show that Arg143 serves as a kinetically significant anion binding (KISAB) site that is, by definition, sensitive to salt concentration and critical to the conformational change necessary to induce iron release from the N-lobe of hTF (in the absence of the TFR). This is the first identification of an authentic KISAB site in the N-lobe of hTF. The effect of the single R143A mutation on the kinetic profile of iron release provides a dramatic illustration of the dynamic nature of hTF.
Efficient delivery of iron is critically dependent on the binding of diferric human serum transferrin (hTF) to its specific receptor (TFR) on the surface of actively dividing cells. Internalization of the complex into an endosome precedes iron removal. The return of hTF to the blood to continue the iron delivery cycle relies on the maintenance of the interaction between apohTF and the TFR after exposure to endosomal pH (≤ 6.0). Identification of the specific residues accounting for the pH-sensitive nanomolar affinity with which hTF binds to TFR throughout the cycle is important to fully understand the iron delivery process. Alanine substitution of eleven charged hTF residues identified by available structures and modeling studies allowed evaluation of the role of each in (1) binding of hTF to the TFR and (2) in TFR-mediated iron release. Six hTF mutants (R50A, R352A, D356A, E357A, E367A and K511A) competed poorly with biotinylated diferric hTF for binding to TFR. In particular, we show that Asp356 in the C-lobe of hTF is essential to the formation of a stable hTF/TFR complex: mutation of Asp356 in the monoferric C-lobe hTF background prevented the formation of the stoichiometric 2:2 (hTF:TFR monomer) complex. Moreover, mutation of three residues (Asp356, Glu367 and Lys511), whether in the diferric or monoferric C-lobe hTF, significantly affected iron release when in complex with the TFR. Thus, mutagenesis of charged hTF residues has allowed identification of a number of residues that are critical to formation of and iron release from the hTF/TFR complex.
Human serum transferrin (hTF) is a bilobal glycoprotein that transports iron to cells. At neutral pH, diferric hTF binds with nM affinity to the transferrin receptor (TFR) on the cell surface. The complex is taken into the cell where, at the acidic pH of the endosome (~pH 5.6), iron is released. Since iron coordination strongly quenches the intrinsic tryptophan fluorescence of hTF, the increase in the fluorescent signal reports the rate constant(s) of iron release. At pH 5.6, the TFR considerably enhances iron release from the C-lobe (with little effect on iron release from the N-lobe). The recombinant soluble TFR is a dimer with 11 tryptophan residues per monomer. In the hTF/TFR complex these residues could contribute to and compromise the readout ascribed to iron release from hTF. We report that compared to FeC hTF alone, the increase in the fluorescent signal from the preformed complex of FeC hTF and the TFR at pH 5.6 is significantly quenched (75%). To dissect the contributions of hTF and the TFR to the change in fluorescence, 5-hydroxytryptophan was incorporated into each using our mammalian expression system. Selective excitation of the samples at 280 or 315 nm shows that the TFR contributes little or nothing to the increase in fluorescence when ferric iron is released from FeC hTF. Quantum yield determinations of TFR, FeC hTF and the FeC hTF/TFR complex strongly support our interpretation of the kinetic data.
Metalloproteins; protein-receptor interaction; tryptophan analogues; tryptophan fluorescence; stopped-flow kinetics; BHK cells
Neisseria meningitidis, the causative agent of bacterial meningitis, acquires the essential element iron from the host glycoprotein transferrin (Tf) during infection via a surface Tf receptor system composed of proteins TbpA and TbpB. Here in we present the crystal structures of TbpB from N. meningitidis, in its apo form and in complex with human Tf (hTf). The structure reveals how TbpB sequesters hTf and initiates iron release from hTf.
PMID: 22343719 CAMSID: cams4091
Iron release from human serum transferrin (hTF) has been studied extensively; however, the molecular details of the mechanism(s) remain incomplete. This is in part due to the complexity of this process, which is influenced by lobe–lobe interactions, the transferrin receptor (TFR), the salt effect, the presence of a chelator, and acidification within the endosome, resulting in iron release. The present work brings together many of the concepts and assertions derived from previous studies in a methodical, uniform, and visual manner. Examination of earlier work reveals some uncertainty due to sample and technical limitations. We have used a combination of steady-state fluorescence and urea gels to evaluate the effect of conformation, pH, time, and the soluble portion of the TFR (sTFR) on iron release from each lobe of hTF. The use of authentic recombinant monoferric and locked species removes any possibility of cross-contamination by acquisition of iron. Elimination of detergent by use of the sTFR provides a further technical advantage. We find that iron release from the N-lobe is very sensitive to the conformation of the C-lobe, but is insensitive to the presence of the sTFR or to changes in pH (between 5.6 and 6.4). Specifically, when the cleft of the C-lobe is locked, the urea gels indicate that only about half of the iron is completely removed from the cleft of the N-lobe. Iron release from the C-lobe is most affected by the presence of the sTFR and changes in pH, but is unaffected by the conformation of the N-lobe. A model for iron release from diferric hTF is provided to delineate our findings.
Cooperativity; Urea gels; Steady-state tryptophan fluorescence; Transferrin/transferrin receptor complex; Iron-release model
The Fe3+ binding protein human serum transferrin (hTF) is well known for its role in cellular iron delivery via the transferrin receptor (TFR). A new application is the use of hTF as a therapy and targeted drug delivery system for a number of diseases. Recently, production of hTF in plants has been reported; such systems provide a relatively inexpensive, animal-free (eliminating potential contamination by animal pathogens) method to produce large amounts of recombinant proteins for such biopharmaceutical applications. Specifically, the production of Optiferrin™ (hTF produced in rice, Oryza sativa, from InVitria) has been shown to yield large amounts of functional protein for use in culture medium for cellular iron delivery to promote growth. In the present work we describe further purification (by gel filtration) and characterization of hTF produced in rice (purified Optiferrin™) to determine its suitability in biopharmaceutical applications. The spectral, mass spectrometric, urea gel and kinetic analysis shows that purified Optiferrin™ is similar to recombinant nonglycosylated N-His tagged hTF expressed by baby hamster kidney cells and/or serum derived glycosylated hTF. Additionally, in a competitive immunoassay, iron-loaded Optiferrin™ is equivalent to iron-loaded N-His hTF in its ability to bind to the soluble portion of the TFR immobilized in an assay plate. As an essential requirement for any functional hTF, both lobes of purified Optiferrin™ bind Fe3+ tightly yet reversibly. Although previously shown to be capable of delivering Fe3+ to cells, the kinetics of iron release from iron-loaded Optiferrin™/sTFR and iron-loaded N-His hTF/sTFR complexes differ somewhat. We conclude that the purified Optiferrin™ might be suitable for consideration in biopharmaceutical applications.
human serum transferrin; Optiferrin™; recombinant transferrin; mass spectrometry; iron delivery; kinetics; transferrin receptor
Transferrin (TF) plays a critical physiological role in cellular iron delivery via the transferrin receptor (TFR)-mediated endocytosis pathway in nearly all eukaryotic organisms. Human serum TF (hTF) is extensively used as an iron-delivery vehicle in various mammalian cell cultures for production of therapeutic proteins, and is also being explored for use as a drug carrier to treat a number of diseases by employing its unique TFR-mediated endocytosis pathway. With the increasing concerns over the risk of transmission of infectious pathogenic agents of human plasma-derived TF, recombinant hTF is preferred to use for these applications. Here, we carry out comparative studies of the TFR binding, TFR-mediated endocytosis and cellular iron delivery of recombinant hTF from rice (rhTF), and evaluate its suitability for biopharmaceutical applications.
Through a TFR competition binding affinity assay with HeLa human cervic carcinoma cells (CCL-2) and Caco-2 human colon carcinoma cells (HTB-37), we show that rhTF competes similarly as hTF to bind TFR, and both the TFR binding capacity and dissociation constant of rhTF are comparable to that of hTF. The endocytosis assay confirms that rhTF behaves similarly as hTF in the slow accumulation in enterocyte-like Caco-2 cells and the rapid recycling pathway in HeLa cells. The pulse-chase assay of rhTF in Caco-2 and HeLa cells further illustrates that rice-derived rhTF possesses the similar endocytosis and intracellular processing compared to hTF. The cell culture assays show that rhTF is functionally similar to hTF in the delivery of iron to two diverse mammalian cell lines, HL-60 human promyelocytic leukemia cells (CCL-240) and murine hybridoma cells derived from a Sp2/0-Ag14 myeloma fusion partner (HB-72), for supporting their proliferation, differentiation, and physiological function of antibody production.
The functional similarity between rice derived rhTF and native hTF in their cellular iron delivery, TFR binding, and TFR-mediated endocytosis and intracellular processing support that rice-derived rhTF can be used as a safe and animal-free alternative to serum hTF for bioprocessing and biopharmaceutical applications.
Recombinant human serum transferrin; Transferrin receptor; Endocytosis; Cell growth and proliferation; Antibody production
Transferrin-binding protein B (TbpB) from Neisseria meningitidis binds human transferrin (hTf) at the surface of the bacterial cell as part of the iron uptake process. To identify hTf binding sites within the meningococcal TbpB, defined regions of the molecule were produced in Escherichia coli by a translational fusion expression system and the ability of the recombinant proteins (rTbpB) to bind peroxidase-conjugated hTf was characterized by Western blot and dot blot assays. Both the N-terminal domain (amino acids [aa] 2 to 351) and the C-terminal domain (aa 352 to 691) were able to bind hTf, and by a peptide spot synthesis approach, two and five hTf binding sites were identified in the N- and C-terminal domains, respectively. The hTf binding activity of three rTbpB deletion variants constructed within the central region (aa 346 to 543) highlighted the importance of a specific peptide (aa 377 to 394) in the ligand interaction. Taken together, the results indicated that the N- and C-terminal domains bound hTf approximately 10 and 1000 times less, respectively, than the full-length rTbpB (aa 2 to 691), while the central region (aa 346 to 543) had a binding avidity in the same order of magnitude as the C-terminal domain. In contrast with the hTf binding in the N-terminal domain, which was mediated by conformational epitopes, linear determinants seemed to be involved in the hTf binding in the C-terminal domain. The host specificity for transferrin appeared to be mediated by the N-terminal domain of the meningococcal rTbpB rather than the C-terminal domain, since we report that murine Tf binds to the C-terminal domain. Antisera raised to both N- and C-terminal domains were bactericidal for the parent strain, indicating that both domains are accessible at the bacterial surface. We have thus identified hTf binding sites within each domain of the TbpB from N. meningitidis and propose that the N- and C-terminal domains together contribute to the efficient binding of TbpB to hTf with their respective affinities and specificities for determinants of their ligand.
Derivatives of human transferrin (hTf) with removed or modified N-linked oligosaccharides were compared with native hTf with respect to their binding to bacterial hTf receptors from Neisseria meningitidis, N. gonorrhoeae, and Haemophilus influenzae. Partially and fully deglycosylated hTf were prepared by enzymatic deglycosylation with glycopeptidase F and isolated by concanavalin A-Sepharose affinity chromatography. Oligosaccharide-modified hTf was prepared via mild periodate oxidation. Competition and direct binding experiments with the hTf derivatives demonstrated that the hTf oligosaccharides are not essential for binding to the bacterial hTf receptors.
The interaction between gold-labelled human transferrin (Au-HTF) with live meningococci after growth in vivo or in different in vitro conditions was examined by electron microscopy to localize and quantify the numbers of HTF-binding sites on the cell surface. It was clearly demonstrated that HTF binds to the surface of live meningococci (of different serogroups and serotypes) after growth in either iron-sufficient or iron-restricted cultures, although the degree of labelling was always higher (2- to 35-fold) in the latter case. The commensal Neisseria polysaccharea behaved similarly. Ultrathin sections showed that Au-HTF was localized predominantly on the outer membrane of the cells and vesicles, with hardly any internalization. Au-HTF labelling on meningococci was significantly reduced after incubation with unlabelled HTF or with rabbit antiserum containing antibodies against transferrin-binding proteins (TBPs), demonstrating the specificity of the interaction. These sera also blocked binding between HTF and outer membrane proteins on Western immunoblots. Direct evidence of the expression of the TBPs (Western blots) and localization of the HTF receptor (electron microscopy) on in vivo-grown meningococci was obtained from organisms derived without laboratory culturing from the cerebrospinal fluid of a patient. There was considerable cell-to-cell variation in the amount of labelling present on cells of the same sample (in vitro- or in vivo-grown organisms) and between different strains. The degree of binding varied with time of incubation of the cells with Au-HTF. The gold particles frequently formed discrete circles on the cell surfaces of the in vitro-grown organisms; these circles appear to be associated with outer membrane vesicle formation. The results show that the TBPs, which form part of the active components of the HTF receptor(s), are expressed in vivo and are surface exposed and immunogenic and that antibodies against them can interfere with the HTF binding of the meningococcal cells, which may affect iron utilization. This study further supports the concept of regarding the TBPs as future vaccine candidates.
A sequence-based prediction method was employed to identify three ligand-binding domains in transferrin-binding protein B (TbpB) of Neisseria meningitidis strain B16B6. Site-directed mutagenesis of residues located in these domains has led to the identification of two domains, amino acids 53 to 57 and 240 to 245, which are involved in binding to human transferrin (htf). These two domains are conserved in an alignment of different TbpB sequences from N. meningitidis and Neisseria gonorrhoeae, indicating a general functional role of the domains. Western blot analysis and BIAcore and isothermal titration calorimetry experiments demonstrated that site-directed mutations in both binding domains led to a decrease or abolition of htf binding. Analysis of mutated proteins by circular dichroism did not provide any evidence for structural alterations due to the amino acid replacements. The TbpB mutant R243N was devoid of any htf-binding activity, and antibodies elicited by the mutant showed strong bactericidal activity against the homologous strain, as well as against several heterologous tbpB isotype I strains.
The mouse CpG-rich island HTF9 harbours the divergent RNA initiation sites shared by two genes that are both expressed in a housekeeping fashion. In this work we have analyzed the architecture of the HTF9 promoter. Gel shift assays were first employed to locate nuclear factor-binding sites within HTF9. Multiple protein-binding sites were identified across a 500 bp-long region, two of which appear to interact with novel factors. Deletion analysis was used to determine the requirements for the different sites in transient expression of a CAT reporter gene. Although multiple elements contributed to the overall promoter strength in each orientation, extensive deletions failed to affect the basal level of transcription from HTF9 in either direction. Thus, only a subset of elements is necessary to activate transcription from HTF9. Functional redundancy may be a general feature of housekeeping CpG-rich promoters.
Transferrin (Tf) expression is enhanced by aging and inflammation in humans. We investigated the role of transferrin in glial protection.
We generated transgenic mice (Tg) carrying the complete human transferrin gene on a C57Bl/6J genetic background. We studied human (hTf) and mouse (mTf) transferrin localization in Tg and wild-type (WT) C57Bl/6J mice using immunochemistry with specific antibodies. Müller glial (MG) cells were cultured from explants and characterized using cellular retinaldehyde binding protein (CRALBP) and vimentin antibodies. They were further subcultured for study. We incubated cells with FeCl3-nitrilotriacetate to test for the iron-induced stress response; viability was determined by direct counting and measurement of lactate dehydrogenase (LDH) activity. Tf expression was determined by reverse transcriptase-quantitative PCR with human- or mouse-specific probes. hTf and mTf in the medium were assayed by ELISA or radioimmunoassay (RIA), respectively.
mTf was mainly localized in retinal pigment epithelium and ganglion cell layers in retina sections of both mouse lines. hTf was abundant in MG cells. The distribution of mTf and hTf mRNA was consistent with these findings. mTf and hTf were secreted into the medium of MG cell primary cultures. Cells from Tg mice secreted hTf at a particularly high level. However, both WT and Tg cell cultures lose their ability to secrete Tf after a few passages. Tg MG cells secreting hTf were more resistant to iron-induced stress toxicity than those no longer secreted hTf. Similarly, exogenous human apo-Tf, but not human holo-Tf, conferred resistance to iron-induced stress on MG cells from WT mice.
hTf localization in MG cells from Tg mice was reminiscent of that reported for aged human retina and age-related macular degeneration, both conditions associated with iron deposition. The role of hTf in protection against toxicity in Tg MG cells probably involves an adaptive mechanism developed in neural retina to control iron-induced stress.
Transferrin-binding protein B (TbpB) is one component of a bipartite receptor in several gram-negative bacterial species that binds host transferrin and mediates the uptake of iron for growth. Transferrin and TbpB are both bilobed proteins, and the interaction between these proteins seems to involve similar lobe-lobe interactions. Synthetic overlapping peptide libraries representing the N lobe of TbpB from Moraxella catarrhalis were prepared and probed with labeled human transferrin. Transferrin-binding peptides were localized to six different regions of the TbpB N lobe, and reciprocal experiments identified six different regions of the C lobe of transferrin that bound TbpB. Truncations of the N lobe of TbpB that sequentially removed each transferrin-binding determinant were used to probe an overlapping peptide library of the C lobe of human transferrin. The removal of each TbpB N-lobe transferrin-binding determinant resulted in a loss of reactivity with peptides from the synthetic peptide library representing the C lobe of transferrin. Thus, individual peptide-peptide interactions between ligand and receptor were identified. A structural model of human transferrin was used to map surface regions capable of binding to TbpB.
Plutonium is a toxic synthetic element with no natural biological function, but it is strongly retained by humans when ingested. Using small angle X-ray scattering, receptor binding assays, and synchrotron X-ray fluorescence microscopy we find that rat adrenal gland (PC12) cells can acquire plutonium in vitro through the major iron acquisition pathway, receptor-mediated endocytosis of the iron transport protein serum transferrin; however only one form of the plutonium-transferrin complex is active. Low-resolution solution models of plutonium-loaded transferrins derived from small angle scattering demonstrate that only transferrin with plutonium bound in the protein’s C-terminal lobe and iron bound in the N-lobe (PuCFeNTf) adopts the proper conformation for recognition by the transferrin receptor protein. Although the metal binding site in each lobe contains the same donors in the same configuration and both lobes are similar, the differences between transferrin’s two lobes act to restrict, but not eliminate, cellular Pu uptake.
Staphylococcus aureus and Staphylococcus epidermidis both recognize and bind the human iron-transporting glycoprotein, transferrin, via a 42-kDa cell surface protein receptor. In an iron-deficient medium, staphylococcal growth can be promoted by the addition of human diferric transferrin but not human apotransferrin. To determine whether the staphylococcal transferrin receptor is involved in the removal of iron from transferrin, we employed 6 M urea–polyacrylamide gel electrophoresis, which separates human transferrin into four forms (diferric, monoferric N-lobe, and monoferric C-lobe transferrin and apotransferrin). S. aureus and S. epidermidis but not Staphylococcus saprophyticus (which lacks the transferrin receptor) converted diferric human transferrin into its apotransferrin form within 30 min. During conversion, iron was removed sequentially from the N lobe and then from the C lobe. Metabolic poisons such as sodium azide and nigericin inhibited the release of iron from human transferrin, indicating that it is an energy-requiring process. To demonstrate that this process is receptor rather than siderophore mediated, we incubated (i) washed staphylococcal cells and (ii) the staphylococcal siderophore, staphyloferrin A, with porcine transferrin, a transferrin species which does not bind to the staphylococcal receptor. While staphyloferrin A removed iron from both human and porcine transferrins, neither S. aureus nor S. epidermidis cells could promote the release of iron from porcine transferrin. In competition binding assays, both native and recombinant N-lobe fragments of human transferrin as well as a naturally occurring human transferrin variant with a mutation in the C-lobe blocked binding of 125I-labelled transferrin. Furthermore, the staphylococci removed iron efficiently from the iron-loaded N-lobe fragment of human transferrin. These data demonstrate that the staphylococci efficiently remove iron from transferrin via a receptor-mediated process and provide evidence to suggest that there is a primary receptor recognition site on the N-lobe of human transferrin.
Retinal degeneration has been associated with iron accumulation in age-related macular degeneration (AMD), and in several rodent models that had one or several iron regulating protein impairments. We investigated the iron concentration and the protective role of human transferrin (hTf) in rd10 mice, a model of retinal degeneration.
The proton-induced X-ray emission (PIXE) method was used to quantify iron in rd10 mice 2, 3, and 4 weeks after birth. We generated mice with the β-phosphodiesterase mutation and hTf expression by crossbreeding rd10 mice with TghTf mice (rd10/hTf mice). The photoreceptor loss and apoptosis were evaluated by terminal deoxynucleotidyl transferase dUTP nick end labeling in 3-week-old rd10/hTf mice and compared with 3-week-old rd10 mice. The neuroprotective effect of hTf was analyzed in 5-day-old rd10 mice treated by intraperitoneal administration with hTf for up to 25 days. The retinal hTf concentrations and the thickness of the outer nuclear layer were quantified in all treated mice at 25 days postnatally.
PIXE analysis demonstrated an age-dependent iron accumulation in the photoreceptors of rd10 mice. The rd10/hTf mice had the rd10 mutation, expressed high levels of hTf, and showed a significant decrease in photoreceptor death. In addition, rd10 mice intraperitoneally treated with hTf resulted in the retinal presence of hTf and a dose-dependent reduction in photoreceptor degeneration.
Our results suggest that iron accumulation in the retinas of rd10 mutant mice is associated with photoreceptor degeneration. For the first time, the enhanced survival of cones and rods in the retina of this model has been demonstrated through overexpression or systemic administration of hTf. This study highlights the therapeutic potential of Tf to inhibit iron-induced photoreceptor cell death observed in degenerative diseases such as retinitis pigmentosa and age-related macular degeneration.
Transferrin receptor (TfR, CD71) has long been therapeutic target due to its over-expression on many malignant tissues. In this study, PRINT® nanoparticles were conjugated with TfR ligands for targeted drug delivery. Cylindrical poly(ethylene glycol)-based PRINT nanoparticles (diameter [d] = 200 nm, height [h] = 200 nm) labeled with transferrin receptor antibody (NP-OKT9) or human transferrin (NP-hTf), showed highly specific TfR-mediated uptake by all human tumor cell lines tested, relative to negative controls (IgG1 for OKT9 or bovine transferrin (bTf) for hTf). The targeting efficiency was dependent on particle concentration, ligand density, dosing time and cell surface receptor expression level. Interestingly, NP-OKT9 or NP-hTf showed little cytotoxicity on all solid tumor cell lines tested but were very toxic to Ramos B-cell lymphoma, whereas free OKT9 or hTf was not toxic. There was a strong correlation between TfR ligand density on particle surface and cell viability and particle uptake. NP-OKT9 and NP-hTf were internalized into acidic intracellular compartments but were not localized in EEA1 enriched early endosomes or lysosomes. Elevated caspase 3/7 activity indicates activation of apoptosis pathways upon particle treatment. Supplementation of iron suppressed the toxicity of NP-OKT9 but not NP-hTf, suggesting different mechanisms by which NP-hTf and NP-OKT9 exerts cytotoxicity on Ramos cells. Based on such an observation, the complex role of multivalency in nanoparticles is discussed. In addition, our data clearly reveal that one must be careful in making claims of “lack of toxicity” when a targeting molecule is used on nanoparticles and also raise concerns for unanticipated off-target effects when one is designing targeted chemotherapy nano-delivery agents.
Transferrin is an enigmatic metalloprotein, which exhibits a profound conformational change upon binding of ferric ion and a synergistic anion (oxalate or carbonate). While the apo- and holo-forms of the protein have well-defined and stable conformations termed “open” and “closed,” certain aspects of transferrin behavior imply the existence of alternative protein states. In this work hydrogen/deuterium exchange was used in combination with mass spectrometry to map solvent-accessible surfaces of the iron-bound and iron-free forms of the N-lobe of human serum transferrin at both neutral and endosomal pH. While the deuterium uptake is significantly decelerated in the iron-bound state of the protein (compared to the apo-form) at neutral pH, the changes are distributed very unevenly across the protein sequence. Protein segments exhibiting most noticeable gain in protection map onto the inter-domain cleft region housing the iron-binding site. At the same time, protection levels of segments located in the bulk of the protein are largely unaffected by the presence of the metal. These observations are fully consistent with the notion of a metal-induced switch from the open to the closed conformation with solvent-inaccessible inter-domain cleft. However, differences in the exchange behavior between the apo- and holo-forms of transferrin become much less noticeable at endosomal pH, including the segments located in the inter-domain cleft region. Intriguingly, a significant patch in the cleft region becomes slightly less protected in the presence of the metal, suggesting that the holo-protein exists in the open conformation under these slightly acidic conditions. The existence of a non-canonical state of holo-transferrin was postulated several years ago; however, this work provides for the first time conclusive evidence that such alternative states are indeed populated in solution.
Metal transport; transferrin; protein conformation; hydrogen/deuterium exchange; mass spectrometry
Neisseria gonorrhoeae is capable of iron utilization from human transferrin in a receptor-mediated event. Transferrin-binding protein 1 (Tbp1) and Tbp2 have been implicated in transferrin receptor function, but their specific roles in transferrin binding and transferrin iron utilization have not yet been defined. We utilized specific gonococcal mutants lacking Tbp1 or Tbp2 to assess the relative transferrin-binding properties of each protein independently of the other. The apparent affinities of the wild-type transferrin receptor and of Tbp1 and Tbp2 individually were much higher than previously estimated for the gonococcal receptor and similar to the estimates for the mammalian transferrin receptor. The binding parameters of both of the mutants were distinct from those of the parent, which expressed two transferrin-binding sites. Tbp2 discriminated between ferrated transferrin and apotransferrin, while Tbp1 did not. Results of transferrin-binding affinity purification, and protease accessibility experiments were consistent with the hypothesis that Tbp1 and Tbp2 interact in the wild-type strain, although both proteins were capable of binding to transferrin independently when separated in the mutants. The presence of Tbp1 partially protected Tbp2 from trypsin proteolysis, and Tbp2 also protected Tbp1 from trypsin exposure. Addition of transferrin to wild-type but not mutant cells protected Tbp1 from trypsin but increased the trypsin susceptibility of Tbp2. These observations indicate that Tbp1 and Tbp2 function together in the wild-type strain to evoke binding conformations that are distinct from those expressed by the mutants lacking either protein.