Production and characterization of chimeric transferrins for the determination of the binding domains for bacterial transferrin receptors

The function of Eriocheir sinensis transferrin and iron in Spiroplasma eriocheiris infection

Transferrin, a member of the iron binding superfamily protein, plays an extremely important role in the transport of iron in the biological process of cells. The result of preliminary proteomic study on E. sinensis hemocytes infected Spiroplasma eriocheiris showed the expression of transferrin (EsTF) and ferrin (EsFe) significantly changed. In addition, other reports have confirmed that transferrin, ferritin and iron are involved in the immune response of hosts. In order to validate the immune function of EsTF, the whole length of EsTF was successfully amplified by the gene cloning and RACE technique. The results showed that the full-length cDNA of the EsTF gene was 2748 bp, including a 2193 bp open reading frame which encodes 730 amino acids. The result of bioinformatics analysis showed EsTF contains two highly conserved TR_FER domains. Evolutionary analysis showed that EsTF has a close genetic relationship with other TFs of invertebrates. In addition, EsTF mRNA was highly transcripted in nerve and intestine tissues, followed by hemocytes. The expression of EsTF, EsFe1 and EsFe2 increased after exogenous supplemental of iron under the concentration of 100 nmol/L in water. After exogenous supplement of iron and injection with S. eriocheiris, these three gene transcription of mRNA levels were higher than that of PBS group, while lower than the S. eriocheiris group and the iron group. Besides, the copy number of S. eriocheiris in the experimental group was significantly reduced, and the death rate decreased. As can be seen, iron made transferrin and ferritin return to normal levels during the infection of S. eriocheiris and help the host maintain normal immunity levels to resist S. eriocheiris. These results further demonstrated that EsTF, EsFe1, EsFe2 and iron play a role in the immune defense mechanism of the crabs to resist S. eriocheiris infection.

Metallomic study on the metabolism of RAPTA-C and cisplatin in cell culture medium and its impact on cell accumulation

The different extracellular speciation of cisplatin and the organoruthenium developmental anticancer agent RAPTA-C impacts the accumulation in cancer cells.

cDNA cloning and expression characterization of serum transferrin gene from oriental weatherfish Misgurnus anguillicaudatus

In this study, the cDNA coding serum transferrin (stf) of Misgurnus anguillicaudatus (mastf) was cloned. mastf cDNA is composed of 2326 bp with a 2007 bp open reading frame encoding 668 amino acids. The deduced Mastf protein consists of a signal peptide, two lobes (N and C-lobes) and signature motifs of transferrin (Tf) family. The results of tissue distribution indicated that mastf mRNA was predominantly expressed in the liver. The results indicate that the mastf expression increased significantly in liver, blood, spleen and head kidney after the challenge with Aeromonas sobria, acting as a positive acute protein, suggesting that mastf is related to the immune response. The cloning and expression analysis of mastf further demonstrates the evolutionary conservation of Stf and immune function in vertebrates.

Effect-based proteomic detection of growth promoter abuse

Unregulated growth promoter use in food-producing animals is an issue of concern both from food safety and animal welfare perspectives. However, the monitoring of such practices is analytically challenging due to the concerted actions of users to evade detection. Techniques based on the monitoring of biological responses to exogenous administrations have been proposed as more sensitive methods to identify treated animals. This study has, for the first time, profiled plasma proteome responses in bovine animals to treatment with nortestosterone decanoate and 17β-oestradiol benzoate, followed by dexamethasone administration. Two-dimensional fluorescence differential in-gel electrophoresis analysis revealed a series of hepatic and acute-phase proteins within plasma whose levels were up- or down-regulated within phases of the treatment regime. Surface plasmon resonance (SPR) immuno-assays were developed to quantify responses of identified protein markers during the experimental treatment study with a view to developing methods which can be used as screening tools for growth promoter abuse detection. SPR analysis demonstrated the potential for plasma proteins to be used as indicative measures of growth promoter administrations and concludes that the sensitivity and robustness of any detection approach based on plasma proteome analysis would benefit from examination of a range of proteins representative of diverse biological processes rather being reliant on specific individual markers.

Transferrin-mediated cellular iron delivery

Essential to iron homeostasis is the transport of iron by the bilobal protein human serum transferrin (hTF). Each lobe (N- and C-lobe) of hTF forms a deep cleft which binds a single Fe(3+). Iron-bearing hTF in the blood binds tightly to the specific transferrin receptor (TFR), a homodimeric transmembrane protein. After undergoing endocytosis, acidification of the endosome initiates the release of Fe(3+) from hTF in a TFR-mediated process. Iron-free hTF remains tightly bound to the TFR at acidic pH; following recycling back to the cell surface, it is released to sequester more iron. Efficient delivery of iron is critically dependent on hTF/TFR interactions. Therefore, identification of the pH-specific contacts between hTF and the TFR is crucial. Recombinant protein production has enabled deconvolution of this complex system. The studies reviewed herein support a model in which pH-induced interrelated events control receptor-stimulated iron release from each lobe of hTF.

Characterization of the gilthead seabream (Sparus aurata L.) transferrin gene: genomic structure, constitutive expression and SNP variation

Transferrin (Tf) is a multi-function protein with a central role in iron metabolism, and it is this function that is associated with a role in the innate immune system response. The clear link between Tf and immune defense mechanism lead to propose Tf as a candidate gene for disease resistance. In this study, genomic and cDNA sequences of Tf gene in gilthead seabream (Sparus aurata L.) (SaTf gene), were identified and characterized. SaTf gene structure consists of a coding region of 2076 nucleotides divided into 17 exons and a no-coding region that includes 16 introns and spans 5495 nucleotides. The deduced Tf protein for gilthead seabream is composed of 691 amino acids and consists of an initial peptide and two lobes (N- and C-lobes). This gene structure is similar to that of previously described Tf genes in other fish species. RT-PCR analyses carried out in different tissues and two developmental stages showed tissue-and stage-specific Tf expression in gilthead seabream. Finally, by sequencing the transferrin genomic sequences of 20 unrelated seabreams, 31 SNPs were identified. These data allowed the estimation of the frequency of nucleotide substitution in the SaTf gene as 1SNP per 253 bp. SNPs were detected in different regions of the genomic sequence but they were mainly localized in non-coding regions, specifically, SNP frequency in non-coding regions was fifteen-fold higher than within coding regions.

Three novel SNPs of the bovine Tf gene in Chinese native cattle and their associations with milk production traits

Transferrin (Tf) is a β-globulin protein that transports iron ions in mammalian cells. It contributes to innate immunity to microbial pathogens, primarily by limiting microbial access to iron. Thus, polymorphisms present in bovine Tf could potentially underlie inherited differences in mastitis resistance and milk production traits. We detected three novel single-nucleotide polymorphisms of the Tf gene in Chinese native cattle by screening for genetic variation of Tf in 751 individuals of three Chinese cattle breeds, namely China Holstein, Luxi Yellow and Bohai Black, using PCR-RFLP and DNA sequencing techniques. The three new SNPs, g.-1748G>A ss250608649, g.13942T>C ss250608650, and g.14037A>G ss250608651, had allele frequencies of 85.9, 86.3 and 92.5%, 64.5, 73.3 and 65.0%, and 67.6, 73.7 and 60.0%, respectively. SNP g.-1748G>A was located in the 5' flanking region of Tf. SNP g.14037A>G was located in intron 8 of Tf. SNP g.13942T>C, located in exon 8 of Tf, was a synonymous mutation (TTA > CTA), encoding a leucine (326 aa) in the Tf protein. Associations of the Tf SNPs with milk traits were also analyzed. Significant (P < 0.05) relationships among the Tf polymorphisms, somatic cell scores (SCS), and milk productive traits were observed. Cows with genotypes TT (g.13942T>C), GG (g.-1748G>A) and AG (g.14037A>G) had a lower SCS and higher protein levels and 305-day milk yield. Nineteen combinations of different haplotypes from the three SNPs were identified in Chinese Holstein cattle. The haplotype combination ATA/GCA, GCA/GCA and GCG/ GTA was dominant in cows with a lower SCS, a higher protein level and a higher 305-day milk yield, respectively. Moreover, the gene expression level of Tf was higher in mastitis-affected mammary tissues than in normal mammary tissues. These results suggest that the Tf gene affects milk production, as well as mastitis-resistance traits, in Chinese Holsteins.

The N1 domain of human lactoferrin is required for internalization by caco-2 cells and targeting to the nucleus

Human lactoferrin (hLf) has been shown to interact with cells from the Caco-2 human small intestinal cell line. There currently is little information about the molecular details of its interaction. As a first step toward detailed characterization of this interaction, we used a series of Lf chimeras to analyze which part of Lf is responsible for the interaction with Caco-2 cells. Recombinant chimeric proteins consisting of segments of hLf and bovine transferrin (bTf) were produced in a baculovirus-insect cell system and purified by a combination of cation exchange chromatography and immobilized bTf antibody affinity chromatography. Each chimera was labeled with a green fluorescent dye to monitor its interaction with Caco-2 cells. Similarly, the intestinal Lf receptor (LfR), also known as intelectin, was probed with an anti-LfR antibody that was detected with a secondary antibody conjugated with a red-color fluorescent dye. The results demonstrated that chimeric proteins containing the N-lobe or the N1.1 subdomain of Lf bound as well as intact Lf to Caco-2 cells. Confocal microscopy analysis revealed that these proteins, along with the LfR, were internalized and targeted to the nucleus. These results indicate that the N1.1 subdomain of hLf is sufficient for binding, internalization, and targeting to the nucleus of Caco-2 cells.

Molecular correlates of host specialization in Staphylococcus aureus

Background

The majority of Staphylococcus aureus isolates that are recovered from either serious infections in humans or from mastitis in cattle represent genetically distinct sets of clonal groups. Moreover, population genetic analyses have provided strong evidence of host specialization among S. aureus clonal groups associated with human and ruminant infection. However, the molecular basis of host specialization in S. aureus is not understood.

Methodology/Principal Findings

We sequenced the genome of strain ET3-1, a representative isolate of a common bovine mastitis-causing S. aureus clone. Strain ET3-1 encodes several genomic elements that have not been previously identified in S. aureus, including homologs of virulence factors from other Gram-positive pathogens. Relative to the other sequenced S. aureus associated with human infection, allelic variation in ET3-1 was high among virulence and surface-associated genes involved in host colonization, toxin production, iron metabolism, antibiotic resistance, and gene regulation. Interestingly, a number of well-characterized S. aureus virulence factors, including protein A and clumping factor A, exist as pseudogenes in ET3-1. Whole-genome DNA microarray hybridization revealed considerable similarity in the gene content of highly successful S. aureus clones associated with bovine mastitis, but not among those clones that are only infrequently recovered from bovine hosts.

Conclusions/Significance

Whole genome sequencing and comparative genomic analyses revealed a set of molecular genetic features that distinguish clones of highly successful bovine-associated S. aureus optimized for mastitis pathogenesis in cattle from those that infect human hosts or are only infrequently recovered from bovine sources. Further, the results suggest that modern bovine specialist clones diverged from a common ancestor resembling human-associated S. aureus clones through a combination of foreign DNA acquisition and gene decay.

Meningococcal transferrin-binding proteins A and B show cooperation in their binding kinetics for human transferrin

Neisseria meningitidis, a causative agent of bacterial meningitis and septicemia, obtains transferrin-bound iron by expressing two outer membrane-located transferrin-binding proteins, TbpA and TbpB. A novel system was developed to investigate the interaction between Tbps and human transferrin. Copurified TbpA-TbpB, recombined TbpA-TbpB, and individual TbpA and TbpB were reconstituted into liposomes and fused onto an HPA chip (BIAcore). All preparations formed stable monolayers, which, with the exception of TbpB, could be regenerated by removing bound transferrin. The ligand binding properties of these monolayers were characterized with surface plasmon resonance and shown to be specific for human transferrin. Kinetic data for diferric human transferrin binding showed that recombined TbpA-TbpB had K(a) and K(d) values similar to those of copurified TbpA-TbpB. Individual TbpA and TbpB also displayed K(a) values similar to those of copurified TbpA-TbpB, but their K(d) values were one order of magnitude higher. Chemical cross-linking studies revealed that TbpA and TbpB, in the absence of human transferrin, formed large complexes with TbpA as the predominant species. Upon human transferrin binding, a complex was formed with a molecular mass corresponding to that of a TbpB-human transferrin heterodimer as well as a higher-molecular-mass complex of this heterodimer cross-linked to TbpA. This indicates that TbpA and TbpB form a functional meningococcal receptor complex in which there is cooperativity in the human transferrin binding kinetics. However, iron loss from the diferric human transferrin-TbpA-TbpB complex was not greater than that from human transferrin alone, suggesting that additional meningococcal transport components are involved in the process of iron removal.

Genetic variability in ESAG6 genes among Trypanosoma evansi isolates and in comparison to other Trypanozoon members

Bloodstream trypanosomes take up iron needed for their propagation through the transferrin receptor that, in Trypanosoma brucei, is encoded by expression-site-associated genes (ESAGs), ESAG6 and 7 genes located in variant surface glycoprotein expression sites. ESAG6 and 7 genes in different expression sites have been shown to encode transferrin receptors with varying affinities for polymorphic transferrins. T. brucei could cope with the different host transferrins by switching between expression sites. ESAG6- and 7-encoded transferrin receptor appear to be present in Trypanosoma evansi but the genes have not yet been characterized. In this study, we cloned and sequenced different members of ESAG6 genes in seven isolates of T. evansi from geographically distinct localities in Thailand. We assessed the intra- and inter-species genetic variability in the transferrin receptor gene regions involved in transferrin binding and established that T. evansi, like T. brucei, has widely diverse ESAG6 genes. In addition, T. evansi possess a clade of ESAG6 variants not observed in T. brucei and different T. evansi strains share at least two conserved variants. We further noted that T. evansi possesses all the reported T. equiperdum ESAG6 variants as a subset. Our findings depict a correlation between the genetic diversity in the transferrin-binding regions of ESAG6 genes with the broad host range of T. evansi and T. brucei compared to the narrow host range of Trypanosoma equiperdum.

Large cooperativity in the removal of iron from transferrin at physiological temperature and chloride ion concentration

Iron removal from serum transferrin by various chelators has been studied by gel electrophoresis, which allows direct quantitation of all four forms of transferrin (diferric, C-monoferric, N-monoferric, and apotransferrin). Large cooperativity between the two lobes of serum transferrin is found for iron removal by several different chelators near physiological conditions (pH 7.4, 37 degrees C, 150 mM NaCl, 20 mM NaHCO(3)). This cooperativity is manifested in a dramatic decrease in the rate of iron removal from the N-monoferric transferrin as compared with iron removal from the other forms of ferric transferrin. Cooperativity is diminished as the pH is decreased; it is also very sensitive to changes in chloride ion concentration, with a maximum cooperativity at 150 mM NaCl. A mechanism is proposed that requires closure of the C-lobe before iron removal from the N-lobe can be effected; the "open" conformation of the C-lobe blocks a kinetically significant anion-binding site of the N-lobe, preventing its opening. Physiological implications of this cooperativity are discussed.

Iron transport systems in Neisseria meningitidis

Acquisition of iron and iron complexes has long been recognized as a major determinant in the pathogenesis of Neisseria meningitidis. In this review, high-affinity iron uptake systems, which allow meningococci to utilize the human host proteins transferrin, lactoferrin, hemoglobin, and haptoglobin-hemoglobin as sources of essential iron, are described. Classic features of bacterial iron transport systems, such as regulation by the iron-responsive repressor Fur and TonB-dependent transport activity, are discussed, as well as more specific features of meningococcal iron transport. Our current understanding of how N. meningitidis acquires iron from the human host and the vaccine potentials of various components of these iron transport systems are also reviewed.

Bacterial lactoferrin-binding protein A binds to both domains of the human lactoferrin C-lobe

Pathogenic bacteria in the family Neisseriaceae express surface receptors to acquire iron from the mammalian iron-binding proteins. Transferrins and lactoferrins constitute a family of iron-binding proteins highly related in both sequence and structure, yet the bacterial receptors are able to distinguish between these proteins and uphold a strict binding specificity. In order to understand the molecular basis for this specificity, the interaction between human lactoferrin (hLf) and the lactoferrin-binding protein A (LbpA) from Moraxella catarrhalis was studied. A periplasmic expression system was designed for the heterologous expression of LbpA, which enabled the investigation of its binding activity in the absence of lactoferrin-binding protein B (LbpB). To facilitate delineation of the LbpA-binding regions of hLf, chimeric proteins composed of hLf and bovine transferrin were made. Binding studies performed with the chimeric proteins and recombinant LbpA identified two binding regions within the C-terminus of hLf. Furthermore, native LbpA from Moraxella and Neisseria spp. bound the identical spectrum of hybrid proteins as the recombinant receptor, demonstrating a conserved binding interaction with the C-lobe of hLf.

Transferrin and the innate immune response of fish: identification of a novel mechanism of macrophage activation

We have previously demonstrated that a non-cytokine serum protein called transferrin was a primary activating molecule of the goldfish (Carassius auratus) macrophage antimicrobial response. The ability of the enzymatically cleaved forms of this protein to modulate fish macrophage function is novel and may represent a primitive and evolutionary conserved mechanism for the induction of NO response of macrophages. In the present study we confirm our earlier findings using immunoaffinity purified goldfish transferrin from mitogen-stimulated leukocyte supernatants. In addition we demonstrate that: (1). products released by necrotic/damaged cells contain transferrin-cleaving activity; (2). the cleavage site is located within the bridge peptide connecting the two lobes of the transferrin molecule; (3). transferrin is expressed by activated goldfish macrophages but not mitogen-stimulated kidney leukocytes; and (4). addition of transferrin significantly enhanced the killing response of goldfish macrophages exposed to different pathogens or pathogen products (e.g. lipopolysaccharide, Mycobacterium chelonei, Trypanosoma danilewskyi, Aeromonas salmonicida, and Leishmania major). We propose a model of fish macrophage activation that is mediated by a non-cytokine host protein (i.e. transferrin) in combination with highly conserved innate immunity recognition receptors that are almost certain to exist in teleost.

Peptide-peptide interactions between human transferrin and transferrin-binding protein B from Moraxella catarrhalis

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.

The neural cell adhesion molecule regulates cell-surface delivery of G-protein-activated inwardly rectifying potassium channels via lipid rafts

Mice deficient in the neural cell adhesion molecule (NCAM) exhibit increased anxiety and anxiolytic sensitivity to serotonin 5-HT1A receptor agonists. Here, we investigate the relationship between NCAM and 5-HT1A receptor signaling pathways modulating G-protein-activated inwardly rectifying K+ (Kir3) channels. When studying this relationship in cultured hippocampal neurons, we observed that in cells from NCAM-deficient mice, inwardly rectifying K+ (Kir3) currents were increased compared with wild-type controls. Analysis of this modulatory mechanism in Xenopus oocytes and Chinese hamster ovary (CHO) cells revealed that the recombinantly expressed major transmembrane isoforms NCAM140 and NCAM180 specifically reduced inward currents generated by neuronal Kir3.1/3.2 and Kir3.1/3.3 but not by cardiac Kir3.1/3.4 channels. Using fluorescence measurements and surface biotinylation assays, we show that this effect was caused by a reduced surface localization of Kir3 channels. Furthermore, expression of flag-tagged Kir3 channels in cultured neurons of NCAM-deficient mice resulted in a higher transport of these channels into neurites and a higher cell-surface localization compared with wild-type neurons. Neuronal Kir3 channels and NCAM isoforms are associated with cholesterol-rich microdomains (lipid rafts) in CHO cells and in isolated brain membranes. Mutational and pharmacological disruption of the lipid raft association of NCAM140 normalizes surface delivery of channels. We conclude that the transmembrane isoforms of NCAM reduce the transport of Kir3 channels to the cell surface via lipid rafts. Thus, regulation of Kir3 channel activity by NCAM may represent a novel mechanism controlling long-term excitability of neurons.

A 3'-truncated transferrin messenger RNA is expressed in rat testicular germ cells

Rat germ cells express a 0.9-kilobase (kb) message with a sequence similar to that of the 3' portion of mammalian transferrins. The sequence of this transcript, called hemiferrin, was considered unique, suggesting that it was encoded by a gene different from that of rat transferrin. Difficulties in conducting experiments using hemiferrin sequence primers led us to question the original sequence. Ribonuclease protection assays revealed that the hemiferrin sequence provided protection only for bovine sequences and not for rat mRNA. Conversely, a 3' rat transferrin sequence protected only rat liver and testis RNA sequences and not bovine sequences, indicating that the 0.9-kb transcript in germ cells is a truncated form of rat transferrin. Western analysis and immunoprecipitation of germ cell proteins metabolically radiolabeled in vitro and in vivo failed to detect a protein of the predicted size regardless of whether anti-rat transferrin or anti-hemiferrin antibodies were used. The findings suggest that a foreshortened transcript of the transferrin gene is produced in rat germ cells and that little or no protein is made from that transcript.

Structure-based combinatorial protein engineering (SCOPE)

Presented here is the development a semi-rational protein engineering approach that uses information from protein structure coupled with established DNA manipulation techniques to design and create multiple crossover libraries from non-homologous genes. The utility of structure-based combinatorial protein engineering (SCOPE) was demonstrated by its application to two distantly related members of the X-family of DNA polymerases: rat DNA polymerase beta (Pol beta) and African swine fever virus DNA polymerase X (Pol X). These proteins share similar folds but have low sequence identity, and differ greatly in both size and activity. "Equivalent" subdomain elements of structure were designed on the basis of the tertiary structure of Pol beta and the corresponding regions of Pol X were inferred from homology modeling and sequence alignment analysis. Libraries of chimeric genes with up to five crossovers were synthesized in a series of PCR reactions by employing hybrid oligonucleotides that code for variable connections between structural elements. Genetic complementation in Escherichia coli enabled identification of several novel DNA polymerases with enhanced phenotypes. Both the composition of structural elements and the manner in which they were linked were shown to be essential for this property, indicating the importance of these aspects of design.

The effect of apo-transferrin on bacterial adhesion to biomaterials

Apo-transferrin (apo-Tf), the iron deficient form of Tf, has been identified previously as a potent inhibitor of Staphylococcus epidermidis adhesion to polyurethane surfaces. In this study, the ability of apo-Tf to suppress the adhesion of two other strains of bacteria, namely a Gram-positive Staphylococcus aureus and a Gram-negative Pseudomonas aeruginosa to several biomaterials, including polystyrene, polymethylmethacrylate, and silicone, is documented. The presence of apo-Tf in the medium at 20 microg/ml lowered bacterial adhesion to all tested biomaterials more than fourfold. Moreover, apo-Tf exerted its inhibitory activity even when protein coated surfaces were used as substrates for bacterial adhesion. To emphasize the importance of apo-Tf in the prevention of bacterial adhesion, human serum was depleted of Tf, employing affinity chromatography, and was shown to lose its inhibitory activity toward bacterial adhesion. Upon addition of apo-Tf to Tf-depleted serum, the activity was reestablished, resulting in a marked reduction in the number of bacteria adhered to the surfaces. Following the enzymatic deglycosylation, apo-Tf retained its ability to prevent bacterial adhesion. These results indicate that the carbohydrate moiety does not seem to play a role in this activity. The presented data provide the evidence that the inhibitory activity of apo-Tf is not bacterial strain specific and that the presence of apo-Tf in the medium results in a significant reduction of bacterial adhesion to a variety of neat and/or protein coated surfaces.

The physiological significance of transferrin receptor variations in Trypanosoma brucei

Trypanosoma brucei escapes destruction by the host immune system by regularly replacing its Variant Surface Glycoprotein (VSG) coat. The VSG is expressed in a VSG expression site, together with expression site associated gene (ESAG) 6 and 7, encoding the heterodimeric transferrin receptor (Tf-R). There are around 20 VSG expression sites, and trypanosomes can change the site that is active. Since ESAG6 and 7 in different expression sites differ somewhat in sequence, expression site switching results in the production of a slightly different Tf-R. We have studied the physiological relevance of Tf-R variation for the survival of T. brucei in mammalian sera. Trypanosomes with an active 221 expression site, encoding a Tf-R with a very low affinity for canine Tf (Kd>1 microM), were cultured in canine serum based medium. This resulted in selection of trypanosomes that had switched to the VO2, the 223 or the bR-2 expression site, each encoding a Tf-R with higher affinity for canine Tf than the 221 site Tf-R. Adding bovine Tf to the medium could prevent the switch, indicating that the low uptake of Tf provided the selection against 221 trypanosomes. Horse serum based medium also induced switching to the VO2 expression site, but this was not prevented by bovine Tf. In the presence of physiological concentrations of anti-Tf-R antibody, only a high-affinity Tf allowed the growth of 221 Tf-R expressing trypanosomes. Our results suggest that a high-affinity Tf-R not only ensures efficient Tf uptake, but is also required to allow sufficient iron uptake by the trypanosome in the presence of anti-Tf-R antibodies.

Entamoeba histolytica: transferrin binding proteins

Entamoeba histolytica trophozoites depend on iron for their growth; thus, they must use some host iron-containing molecules to fulfill this requirement. In this work we report that amoebas are able to utilize human holo-Tf as iron source and to recognize it through transferrin binding proteins. By use of an anti-human transferrin antiserum in an immunoblotting assay, two main polypeptides with apparent molecular masses of 70 and 140 kDa were found in total extract of trophozoites cultured in vitro. However, when a monoclonal anti-human transferrin receptor antibody was used, only one band with molecular mass of 140 kDa was observed. Both the human transferrin and the monoclonal antibody recognized a protein on the amoebic surface, demonstrated by confocal microscopy. Furthermore, the complex transferrin-transferrin binding protein was internalized by an endocytic process and probably dissociated inside the cell. This mechanism could be one manner in which E. histolytica acquires iron from the human host transferrin.

Expression, purification, and characterization of recombinant nonglycosylated human serum transferrin containing a C-terminal hexahistidine tag

Attachment of a hexa-His tag is a common strategy in recombinant protein production. The use of such a tag greatly simplifies the purification of the protein from the complex mixture of other proteins in the media or cell extract. We describe the production of two recombinant nonglycosylated human serum transferrins (hTF-NG), containing a factor Xa cleavage site and a hexa-His tag at their carboxyl-terminal ends. One of the constructs comprises the entire coding region for hTF (residues 1-679), while the other lacks the final three carboxyl-terminal amino acids. After insertion of the His-tagged hTFs into the pNUT vector, transfection into baby hamster kidney (BHK) cells, and selection with methotrexate, the secreted recombinant proteins were isolated from the tissue culture medium. Average maximum expression levels of the His-tagged hTFs were about 40 mg/L compared to an average maximum of 50 mg/L for hTF-NG. The first step of purification involved an anion exchange column. The second step utilized a Poros metal chelate column preloaded with copper from which the His-tagged sample was eluted with a linear imidazole gradient. The His-tagged hTFs were characterized and compared to both recombinant hTF-NG and glycosylated hTF from human serum. The identity of each of the His-tagged hTFs constructs was verified by electrospray mass spectroscopy. In summary, the His-tagged hTF constructs simplify the purification of these metal-binding proteins with minimal effects on many of their physical properties. The His-tagged hTFs share many features common to hTF, including reversible iron binding, reactivity with a monoclonal antibody, and presence as a monomer in solution.

Characterization of a single nucleotide polymorphism in the coding sequence of the bovine transferrin gene

A single nucleotide polymorphism was identified in the coding sequence of the bovine transferrin gene. Two alleles (SSCP1 and SSCP2) were detected by SSCP analysis and the mutation point was identified and confirmed by direct sequencing of the PCR products. The relationship between protein and DNA polymorphism was established. Protein variants A, D1 and E correspond to SSCP allele 1 and variant D2 corresponds to SSCP allele 2. DNA sequences from genotypes AA, AE, AD2, D1E, D2E and D2D2 reveal an A/G substitution at position 1455 of the cDNA which causes a Gly/Glu substitution which could be responsible for the mobility difference between D1 and D2 variants. Because of the number of variants, this suggests that other SNPs exist in the bovine transferrin gene. A linkage analysis between the SSCPs and two microsatellites (UWCA46 and CSSM019) mapped the transferrin gene to BTA1. Two-point analysis revealed a tight linkage within the transferrin protein variants and the SSCPs.

Rapid fractionation of bovine transferrin using immobilized gangliosides

Bovine transferrin (BTF) was fractionated from bovine whey using ganglioside affinity chromatography. After loading the immobilized matrix with a 2% whey solution, the matrix was washed with sodium acetate buffer at pH 4 containing 1 M NaCl before elution of BTF with sodium phosphate buffers at pH 7. Concanavalin-A affinity and ion exchange chromatography were used for further purification. The ganglioside column showed a 74.2% BTF recovery from whey and BTF was enriched to 61% purity with ion exchange chromatography. Bovine transferrin was identified by SDS-PAGE and western analysis. The Concanavalin-A affinity and ion exchange chromatography steps enriched BTF in the samples and removed other whey proteins from ganglioside purified fractions. These results indicate that immobilized ganglioside can be used to fractionate BTF from bovine whey. Our novel ganglioside affinity chromatography is rapid and efficient for the fractionation of BTF from whey.

Molecular evolution of transferrin: evidence for positive selection in salmonids

Transferrins are iron-binding proteins that are involved in iron storage and resistance to bacterial disease. Previous work has shown that nonsynonymous-to-synonymous-site substitution ratios (d(n)/d(s) ratios) between transferrin genes from some salmonid species were significantly greater than 1.0, providing evidence for positive selection at the transferrin gene. The purpose of the current study was to put these earlier results in a broader evolutionary context by examining variation among 25 previously published transferrin sequences from fish, amphibians, and mammals. The results of the study show that evidence for positive selection at transferrin is limited to salmonids-d(n)/d(s) ratios estimated for nonsalmonid lineages were generally less than 1.0. Within the salmonids, approximately 13% of the transferrin codon sites are estimated to be subject to positive selection, with an estimated d(n)/d(s) ratio of approximately 7. The three- dimensional locations of some of the selected sites were inferred by comparing these sites to homologous sites in the bovine lactoferrin crystallographic structure. The selected sites generally fall on the outside of the molecule, within and near areas that are bound by transferrin-binding proteins from human pathogenic bacteria. The physical locations of sites estimated to be subject to positive selection support previous speculation that competition for iron from pathogenic bacteria could be the source of positive selection.

A dynamic model of the meningococcal transferrin receptor

Iron is an essential nutrient for all organisms and consequently, the ability to bind transferrin and sequester iron from his source constitutes a distinct advantage to a blood-borne bacterial pathogen. Levels of free iron are strictly limited in human serum, largely through the action of the iron-binding protein transferrin. The acquisition of trasferrin-iron is coincident with pathogenicity among Neisseria species and a limited number of other pathogens of human and veterinary significance. In Neisseria meningitidis, transferrin binding relies on two co-expressed, outer membrane proteins distinct in aspects of both structure and function. These proteins are independently and simultaneously capable of binding human transferrin and both are required for the optimal uptake of iron from this source. It has been established that transferrin-binding proteins (designated TbpA and TbpB) form a discrete, specific complex which may be composed of a transmembrane species (composed of the TbpA dimer) associated with a single surface-exposed lipoprotein (TbpB). This more exposed protein is capable of selectively binding iron-saturated transferrin and the receptor complex has ligand-binding properties which are distinct from either of its components. Previous in vivo analyses of N. gonorrhoeae, which utilizes a closely related transferrin-iron uptake system, indicated that this receptor exists in several conformations influenced in part by the presence (or absence) of transferrin. Here we propose a dynamic model of the meningococcal transferrin receptor which is fully consistent with the current data concerning this subject. We suggest that TbpB serves as the initial binding site for iron-saturated transferrin and brings this ligand close to the associated transmembrane dimer, enabling additional binding events and orientating transferrin over the dual TbpA pores. The antagonistic association of these receptor proteins with a single ligand molecule may also induce conformational change in transferrin, thereby favouring the release of iron. As, in vivo, transferrin may have iron in one or both lobes, this dynamic molecular arrangement would enable iron uptake from either iron-binding site. In addition, the predicted molecular dimensions of the putative TbpA dimer and hTf are fully consistent with these proposals. Given the diverse data used in the formulation of this model and the consistent characteristics of transferrin binding among several significant Gram-negative pathogens, we speculate that such receptor-ligand interactions may be, at least in part, conserved between species. Consequently, this model may be applicable to bacteria other than N. meningitidis.

Identification of sequences in human transferrin that bind to the bacterial receptor protein, transferrin-binding protein B

Alignment of amino-acid sequences from the N-terminal and C-terminal halves of transferrin-binding protein B revealed an underlying bilobed nature with several regions of identity. Based on this analysis, purified recombinant fusion proteins of maltose-binding protein (Mbp) with intact TbpB, its N-terminal half or C-terminal half from the human pathogens Neisseria meningitidis and Moraxella catarrhalis were produced. Solid-phase binding assays and affinity isolation assays demonstrated that the N-terminal and C-terminal halves of TbpB could bind independently to human transferrin (hTf). A solid-phase overlapping synthetic peptide library representing the amino-acid sequence of hTf was probed with soluble, labelled Mbp-TbpB fusions to localize TbpB-binding regions on hTf. An essentially identical series of peptides from domains within both lobes of hTf was recognized by intact TbpB from both organisms, demonstrating a conserved TbpB-hTf interaction. Both halves of TbpB from N. meningitidis bound the same series of peptides, which included peptides from equivalent regions on the two hTf lobes, indicating that TbpB interacts with each lobe of hTf in a similar manner. Mapping of the peptide-binding regions on a molecular model of hTf revealed a series of nearly adjacent surface regions that nearly encircled each lobe. Binding studies with chimeric hTf/bTf transferrins demonstrated that regions in the C-lobe of hTf were preferentially recognized by the N-terminal half of TbpB. Collectively, these results provide evidence that TbpB consists of two lobes, each with distinct yet homologous Tf-binding regions.

X-ray crystallography and mass spectroscopy reveal that the N-lobe of human transferrin expressed in Pichia pastoris is folded correctly but is glycosylated on serine-32

The ferric form of the N-lobe of human serum transferrin (Fe(III)-hTF/2N) has been expressed at high levels in Pichia pastoris. The Fe(III)-hTF/2N was crystallized in the space group P41212, and X-ray crystallography was used to solve the structure of the recombinant protein at 2.5 A resolution. This represents only the second P. pastoris-derived protein structure determined to date, and allows the comparison of the structures of recombinant Fe(III)-hTF/2N expressed in P. pastoris and mammalian cells with serum-derived transferrin. The polypeptide folding pattern is essentially identical in all of the three proteins. Mass spectroscopic analyses of P. pastoris- hTF/2N and proteolytically derived fragments revealed glycosylation of Ser-32 with a single hexose. This represents the first localization of an O-linked glycan in a P. pastoris-derived protein. Because of its distance from the iron-binding site, glycosylation of Ser-32 should not affect the iron-binding properties of hTF/2N expressed in P. pastoris, making this an excellent expression system for the production of hTF/2N.

Discrimination between apo and iron-loaded forms of transferrin by transferrin binding protein B and its N-terminal subfragment

Many pathogens of the Pasteurellaceae and Neisseriaceae possess a surface receptor that binds transferrin (Tf) as an initial step in an iron acquisition process. This receptor is comprised of two proteins, transferrin binding protein A (TbpA) and transferrin binding protein B (TbpB). Since the ability to recognize the iron-loaded form of Tf preferentially would be a useful attribute of these receptors, we examined this property in a number of bacterial species. In solid-phase binding assays with isolated membranes, only the receptor from Moraxella catarrhalis was capable of preferentially binding iron-loaded Tf. In a competitive affinity isolation assay which enabled us to resolve TbpA and TbpB, TbpA from all tested species was shown to bind both apo and iron-loaded Tf. Under these assay conditions TbpB from M. catarrhalis, Haemophilus somnus and Pasteurella haemolytica discriminated between apo and holo Tf, whereas TbpB from Neisseria meningitidis showed no discrimination. The ability of TbpB from N. meningitidis to bind iron-saturated hTf preferentially became evident in a TbpA- background or by using recombinant TbpB. In binding assays with recombinant fusion proteins, both intact TbpB and the N-terminal half of TbpB from all the tested species preferentially bound Fe-loaded Tf, indicating that this may be a conserved mechanism by which these organisms optimize their ability to acquire iron.

The role of transferrin-receptor variation in the host range of Trypanosoma brucei

Trypanosoma brucei is a unicellular parasite transmitted between African mammals by tsetse flies. T. brucei multiplies freely in the bloodstream of many different mammals, and survives by antigenic variation of the main component of its surface coat, variant surface glycoprotein (VSG). Trypanosomes take up transferrin through a heterodimeric transferrin receptor, the genes for which are expressed in telomeric expression sites along with the VSG gene. There are up to 20 of these expression sites per trypanosome nucleus, but usually only one is active at a time. Different expression sites encode transferrin receptors that are similar but not identical. Here we show that these small differences between transferrin receptors can have profound effects on the binding affinity for transferrins from different mammals, and on the ability of trypanosomes to grow in the sera of these mammals. Our results suggest that the ability to switch between different transferrin-receptor genes allows T. brucei to cope with the large sequence diversity in the transferrins of its hosts.

Mosquito transferrin, an acute-phase protein that is up-regulated upon infection

When treated with heat-killed bacterial cells, mosquito cells in culture respond by up-regulating several proteins. Among these is a 66-kDa protein (p66) that is secreted from cells derived from both Aedes aegypti and Aedes albopictus. p66 was degraded by proteolysis and gave a virtually identical pattern of peptide products for each mosquito species. The sequence of one peptide (31 amino acids) was determined and found to have similarity to insect transferrins. By using conserved regions of insect transferrin sequences, degenerate oligonucleotide PCR primers were designed and used to isolate a cDNA clone encoding an A. aegypti transferrin. The encoded protein contained a signal sequence that, when cleaved, would yield a mature protein of 68 kDa. It contained the 31-amino acid peptide, and the 3' end exactly matched a cDNA encoding a polypeptide that is up-regulated when A. aegypti encapsulates filarial worms [Beerntsen, B. T., Severson, D. W. & Christensen, B. M. (1994) Exp. Parasitol. 79, 312-321]. This transferrin, like those of two other insect species, has conserved iron-binding residues in the N-terminal lobe but not in the C-terminal lobe, which also has large deletions in the polypeptide chain, compared with transferrins with functional C-terminal lobes. The hypothesis is developed that this transferrin plays a role similar to vertebrate lactoferrin in sequestering iron from invading organisms and that degradation of the structure of the C-terminal lobe might be a mechanism for evading pathogens that elaborate transferrin receptors to tap sequestered iron.

Identification of human transferrin-binding sites within meningococcal transferrin-binding protein B

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.