Binding and surface exposure characteristics of the gonococcal transferrin receptor are dependent on both transferrin-binding proteins

Rational selection of TbpB variants yields a bivalent vaccine with broad coverage against Neisseria gonorrhoeae

Neisseria gonorrhoeae is an on-going public health problem due in part to the lack of success with efforts to develop an efficacious vaccine to prevent this sexually transmitted infection. The gonococcal transferrin binding protein B (TbpB) is an attractive candidate vaccine antigen. However, it exhibits high levels of antigenic variability, posing a significant obstacle in evoking a broadly protective immune response. Here, we utilize phylogenetic information to rationally select TbpB variants for inclusion into a gonococcal vaccine and identify two TbpB variants that together elicit a highly cross-reactive antibody response against a diverse panel of TbpB variants and clinically relevant gonococcal strains. This formulation performed well in experimental proxies of real-world usage, including eliciting bactericidal activity against diverse gonococcal strains and decreasing the median duration of colonization after vaginal infection in female mice. These data support the use of a combination of TbpB variants for a broadly protective gonococcal vaccine.

Rational selection of TbpB variants elucidates a bivalent vaccine formulation with broad spectrum coverage against Neisseria gonorrhoeae

Neisseria gonorrhoeae is the causative agent of gonorrhea, an on-going public health problem due in part to the lack of success with efforts to develop an efficacious vaccine to prevent this sexually transmitted infection. An attractive candidate vaccine antigen because of its essential function and surface exposure, the gonococcal transferrin binding protein B (TbpB) exhibits high levels of antigenic variability which poses a significant obstacle in evoking a broadly protective vaccine composition. Here, we utilize phylogenetic information to rationally select TbpB variants for inclusion into a potential gonococcal vaccine and identify two TbpB variants that when formulated together elicit a highly cross-reactive antibody response in both rabbits and mice against a diverse panel of TbpB variants and clinically relevant gonococcal strains. Further, this formulation performed well in experimental proxies of real-world usage, including eliciting bactericidal activity against 8 diverse gonococcal strains and decreasing the median duration of colonization after vaginal infection in female mice by two heterologous strains of N. gonorrhoeae . Together, these data support the use of a combination of TbpB variants for a broadly protective gonococcal vaccine.

Investigating the importance of selected surface-exposed loops in HpuB for hemoglobin binding and utilization by Neisseria gonorrhoeae

Neisseria gonorrhoeae is the etiological agent of the sexually transmitted infection gonorrhea. The pathogen is a global health challenge since no protective immunity results from infection, and far fewer treatment options are available with increasing antimicrobial resistance. With no efficacious vaccines, researchers are exploring new targets for vaccine development and innovative therapeutics. The outer membrane TonB-dependent transporters (TdTs) produced by N. gonorrhoeae are considered promising vaccine antigens as they are highly conserved and play crucial roles in overcoming nutritional immunity. One of these TdTs is part of the hemoglobin transport system comprised of HpuA and HpuB. This system allows N. gonorrhoeae to acquire iron from hemoglobin (hHb). In the current study, mutations in the hpuB gene were generated to better understand the structure-function relationships in HpuB. This study is one of the first to demonstrate that N. gonorrhoeae can bind to and utilize hemoglobin produced by animals other than humans. This study also determined that when HpuA is absent, mutations targeting extracellular loop 7 of HpuB led to defective hHb binding and utilization. However, when the lipoprotein HpuA is present, these loop 7 mutants recovered their ability to bind hHb, although the growth phenotype remained significantly impaired. Interestingly, loop 7 contains putative heme-binding motifs and a hypothetical α-helical region, both of which may be important for the use of hHb. Taken together, these results highlight the importance of loop 7 in the functionality of HpuB in binding hHb and extracting and internalizing iron.

Investigating the importance of surface exposed loops in the gonococcal HpuB transporter for hemoglobin binding and utilization

Neisseria gonorrhoeae is the etiological agent of the sexually-transmitted infection gonorrhea and a global health challenge since no protective immunity results from infection and far fewer treatment options are available with increasing antimicrobial resistance. With no efficacious vaccines, researchers are exploring new targets for vaccine development and innovative therapeutics. The outer membrane TonB-dependent transporters (TdTs) produced by N. gonorrhoeae are considered promising antigen targets as they are highly conserved and play crucial roles in overcoming nutritional immunity. One of these TdTs, the hemoglobin transport system comprised of HpuA and HpuB, allows N. gonorrhoeae to acquire iron from hemoglobin (hHb). In the current study, mutations in the hpuB gene were generated to better understand the structure-function relationships in HpuB. This study is one of the first to demonstrate that N. gonorrhoeae can bind to and utilize hemoglobin produced by animals other than humans. This study also determined that when HpuA is absent, mutations targeting extracellular loop 7 of HpuB led to defective hHb binding and utilization. However, when the lipoprotein HpuA is present, these loop 7 mutants recovered their ability to bind hHB, although their growth phenotype remained significantly impaired. Interestingly, loop 7 contains putative heme binding motifs and a hypothetical α-helical region. Taken together, these results highlight the importance of loop 7 in the functionality of HpuB in binding hHb, and extracting and internalizing iron.

Point Mutations in TbpA Abrogate Human Transferrin Binding in Neisseria gonorrhoeae

TonB-dependent transporters (TDTs) are essential proteins for metal acquisition, an important step in the growth and pathogenesis of many pathogens, including Neisseria gonorrhoeae, the causative agent of gonorrhea. There is currently no available vaccine for gonorrhea; TDTs are being investigated as vaccine candidates because they are highly conserved and expressed in vivo. Transferrin binding protein A (TbpA) is an essential virulence factor in the initiation of experimental infection in human males and functions by acquiring iron upon binding to host transferrin (human transferrin [hTf]). The loop 3 helix (L3H) is a helix finger that inserts into the hTf C-lobe and is required for hTf binding and subsequent iron acquisition. This study identified and characterized the first TbpA single-point substitutions resulting in significantly decreased hTf binding and iron acquisition, suggesting that the helix structure is more important than charge for hTf binding and utilization. The tbpA D355P ΔtbpB and tbpA A356P ΔtbpB mutants demonstrated significantly reduced hTf binding and impaired iron uptake from Fe-loaded hTf; however, only the tbpA A356P ΔtbpB mutant was able to grow when hTf was the sole source of iron. The expression of tbpB was able to restore function in all tbpA mutants. These results implicate both D355 and A356 in the key binding, extraction, and uptake functions of gonococcal TbpA.

Stealthy microbes: How Neisseria gonorrhoeae hijacks bulwarked iron during infection

Transition metals are essential for metalloprotein function among all domains of life. Humans utilize nutritional immunity to limit bacterial infections, employing metalloproteins such as hemoglobin, transferrin, and lactoferrin across a variety of physiological niches to sequester iron from invading bacteria. Consequently, some bacteria have evolved mechanisms to pirate the sequestered metals and thrive in these metal-restricted environments. Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection gonorrhea, causes devastating disease worldwide and is an example of a bacterium capable of circumventing human nutritional immunity. Via production of specific outer-membrane metallotransporters, N. gonorrhoeae is capable of extracting iron directly from human innate immunity metalloproteins. This review focuses on the function and expression of each metalloprotein at gonococcal infection sites, as well as what is known about how the gonococcus accesses bound iron.

Acclimation to Nutritional Immunity and Metal Intoxication Requires Zinc, Manganese, and Copper Homeostasis in the Pathogenic Neisseriae

Neisseria gonorrhoeae and Neisseria meningitidis are human-specific pathogens in the Neisseriaceae family that can cause devastating diseases. Although both species inhabit mucosal surfaces, they cause dramatically different diseases. Despite this, they have evolved similar mechanisms to survive and thrive in a metal-restricted host. The human host restricts, or overloads, the bacterial metal nutrient supply within host cell niches to limit pathogenesis and disease progression. Thus, the pathogenic Neisseria require appropriate metal homeostasis mechanisms to acclimate to such a hostile and ever-changing host environment. This review discusses the mechanisms by which the host allocates and alters zinc, manganese, and copper levels and the ability of the pathogenic Neisseria to sense and respond to such alterations. This review will also discuss integrated metal homeostasis in N. gonorrhoeae and the significance of investigating metal interplay.

Generation of Metal-Depleted Conditions for In Vitro Growth of Neisseria gonorrhoeae

Neisseria gonorrhoeae employs high-affinity metal acquisition systems to obtain necessary nutrients, such as iron (Fe) and zinc (Zn) from the environment. Because growth and replication depend upon successful metal acquisition, these high-affinity uptake systems are important virulence factors. Expression of metal acquisition systems is tightly controlled and preferentially expressed under low-metal conditions. Therefore, in order to optimally produce these transport proteins and study them in vitro, growth media must be deployed that mimic low-metal conditions. This chapter describes the chelators, media, and culturing conditions that can generate low-metal in vitro growth conditions.

Structural Basis for Evasion of Nutritional Immunity by the Pathogenic Neisseriae

The pathogenic Neisseria species are human-adapted pathogens that cause quite distinct diseases. Neisseria gonorrhoeae causes the common sexually transmitted infection gonorrhea, while Neisseria meningitidis causes a potentially lethal form of bacterial meningitis. During infection, both pathogens deploy a number of virulence factors in order to thrive in the host. The focus of this review is on the outer membrane transport systems that enable the Neisseriae to utilize host-specific nutrients, including metal-binding proteins such as transferrin and calprotectin. Because acquisition of these critical metals is essential for growth and survival, understanding the structures of receptor-ligand complexes may be an important step in developing preventative or therapeutic strategies focused on thwarting these pathogens. Much can also be learned by comparing structures with antigenic diversity among the transporter sequences, as conserved functional domains in these essential transporters could represent the pathogens' "Achilles heel." Toward this goal, we present known or modeled structures for the transport systems produced by the pathogenic Neisseria species, overlapped with sequence diversity derived by comparing hundreds of neisserial protein sequences. Given the concerning increase in N. gonorrhoeae incidence and antibiotic resistance, these outer membrane transport systems appear to be excellent targets for new therapies and preventative vaccines.

Subversion of nutritional immunity by the pathogenic Neisseriae

The pathogenic Neisseria species, including Neisseria meningitidis and Neisseria gonorrhoeae, are obligate human pathogens that cause significant morbidity and mortality. The success of these pathogens, with regard to causing disease in humans, is inextricably linked to their ability to acquire necessary nutrients in the hostile environment of the host. Humans deploy a significant arsenal of weaponry to defend against bacterial pathogens, not least of which are the metal-sequestering proteins that entrap and withhold transition metals, including iron, zinc and manganese, from invaders. This review will discuss the general strategies that bacteria employ to overcome these metal-sequestering attempts by the host, and then will focus on the relatively uncommon 'metal piracy' approaches utilized by the pathogenic Neisseria for this purpose. Because acquiring metals from the environment is critical to microbial survival, interfering with this process could impede growth and therefore disease initiation or progression. This review will also discuss how interfering with metal uptake by the pathogenic Neisseriae could be deployed in the development of novel or improved preventative or therapeutic measures against these important pathogens.

Biological Functions of the Secretome of Neisseria meningitidis

Neisseria meningitidis is a Gram-negative bacterial pathogen that normally resides as a commensal in the human nasopharynx but occasionally causes disease with high mortality and morbidity. To interact with its environment, it transports many proteins across the outer membrane to the bacterial cell surface and into the extracellular medium for which it deploys the common and well-characterized autotransporter, two-partner and type I secretion mechanisms, as well as a recently discovered pathway for the surface exposure of lipoproteins. The surface-exposed and secreted proteins serve roles in host-pathogen interactions, including adhesion to host cells and extracellular matrix proteins, evasion of nutritional immunity imposed by iron-binding proteins of the host, prevention of complement activation, neutralization of antimicrobial peptides, degradation of immunoglobulins, and permeabilization of epithelial layers. Furthermore, they have roles in interbacterial interactions, including the formation and dispersal of biofilms and the suppression of the growth of bacteria competing for the same niche. Here, we will review the protein secretion systems of N. meningitidis and focus on the functions of the secreted proteins.

Transition metals at the host-pathogen interface: how Neisseria exploit human metalloproteins for acquiring iron and zinc

Transition metals are essential nutrients for all organisms and important players in the host-microbe interaction. During bacterial infection, a tug-of-war between the host and microbe for nutrient metals occurs: the host innate immune system responds to the pathogen by reducing metal availability and the pathogen tries to outmaneuver this response. The outcome of this competition, which involves metal-sequestering host-defense proteins and microbial metal acquisition machinery, is an important determinant for whether infection occurs. One strategy bacterial pathogens employ to overcome metal restriction involves hijacking abundant host metalloproteins. The obligate human pathogens Neisseria meningitidis and N. gonorrhoeae express TonB-dependent transport systems that capture human metalloproteins, extract the bound metal ions, and deliver these nutrients into the bacterial cell. This review highlights structural and mechanistic investigations that provide insights into how Neisseria acquire iron from the Fe(III)-transport protein transferrin (TF), the Fe(III)-chelating host-defense protein lactoferrin (LF), and the oxygen-transport protein hemoglobin (Hb), and obtain zinc from the metal-sequestering antimicrobial protein calprotectin (CP).

The Journey of Lipoproteins Through the Cell: One Birthplace, Multiple Destinations

Bacterial lipoproteins are a very diverse group of proteins characterized by the presence of an N-terminal lipid moiety that serves as a membrane anchor. Lipoproteins have a wide variety of crucial functions, ranging from envelope biogenesis to stress response. In Gram-negative bacteria, lipoproteins can be targeted to various destinations in the cell, including the periplasmic side of the cytoplasmic or outer membrane, the cell surface or the external milieu. The sorting mechanisms have been studied in detail in Escherichia coli, but exceptions to the rules established in this model bacterium exist in other bacteria. In this chapter, we will present the current knowledge on lipoprotein sorting in the cell. Our particular focus will be on the surface-exposed lipoproteins that appear to be much more common than previously assumed. We will discuss the different targeting strategies, provide numerous examples of surface-exposed lipoproteins and discuss the techniques used to assess their surface exposure.

The genes that encode the gonococcal transferrin binding proteins, TbpB and TbpA, are differentially regulated by MisR under iron-replete and iron-depleted conditions

Neisseria gonorrhoeae produces two transferrin binding proteins, TbpA and TbpB, which together enable efficient iron transport from human transferrin. We demonstrate that expression of the tbp genes is controlled by MisR, a response regulator in the two-component regulatory system that also includes the sensor kinase MisS. The tbp genes were up-regulated in the misR mutant under iron-replete conditions but were conversely down-regulated in the misR mutant under iron-depleted conditions. The misR mutant was capable of transferrin-iron uptake at only 50% of wild-type levels, consistent with decreased tbp expression. We demonstrate that phosphorylated MisR specifically binds to the tbpBA promoter and that MisR interacts with five regions upstream of the tbpB start codon. These analyses confirm that MisR directly regulates tbpBA expression. The MisR binding sites in the gonococcus are only partially conserved in Neisseria meningitidis, which may explain why tbpBA was not MisR-regulated in previous studies using this related pathogen. This is the first report of a trans-acting protein factor other than Fur that can directly contribute to gonococcal tbpBA regulation.

Structural analysis of haemoglobin binding by HpuA from the Neisseriaceae family

The Neisseriaceae family of bacteria causes a range of diseases including meningitis, septicaemia, gonorrhoea and endocarditis, and extracts haem from haemoglobin as an important iron source within the iron-limited environment of its human host. Herein we report crystal structures of apo- and haemoglobin-bound HpuA, an essential component of this haem import system. The interface involves long loops on the bacterial receptor that present hydrophobic side chains for packing against the surface of haemoglobin. Interestingly, our structural and biochemical analyses of Kingella denitrificans and Neisseria gonorrhoeae HpuA mutants, although validating the interactions observed in the crystal structure, show how Neisseriaceae have the fascinating ability to diversify functional sequences and yet retain the haemoglobin binding function. Our results present the first description of HpuA's role in direct binding of haemoglobin.

The Neisseriaceae bacteria family extract heame from the haemoglobin of its host, the HpuA protein is part of this system. Here, the authors report crystal structures of apo- and haemoglobin-bound HpuA and analyse mutants to examine the interaction between HpuA and haemoglobin.

Beyond the Crystal Structure: Insight into the Function and Vaccine Potential of TbpA Expressed by Neisseria gonorrhoeae

Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection gonorrhea, is not preventable by vaccination and is rapidly developing resistance to antibiotics. However, the transferrin (Tf) receptor system, composed of TbpA and TbpB, is an ideal target for novel therapeutics and vaccine development. Using a three-dimensional structure of gonococcal TbpA, we investigated two hypotheses, i.e., that loop-derived antibodies can interrupt ligand-receptor interactions in the native bacterium and that the loop 3 helix is a critical functional domain. Preliminary loop-derived antibodies, as well as optimized second-generation antibodies, demonstrated similar modest ligand-blocking effects on the gonococcal surface but different effects in Escherichia coli. Mutagenesis of loop 3 helix residues was employed, generating 11 mutants. We separately analyzed the mutants' abilities to (i) bind Tf and (ii) internalize Tf-bound iron in the absence of the coreceptor TbpB. Single residue mutations resulted in up to 60% reductions in ligand binding and up to 85% reductions in iron utilization. All strains were capable of growing on Tf as the sole iron source. Interestingly, in the presence of TbpB, only a 30% reduction in Tf-iron utilization was observed, indicating that the coreceptor can compensate for TbpA impairment. Complete deletion of the loop 3 helix of TbpA eliminated the abilities to bind Tf, internalize iron, and grow with Tf as the sole iron source. Our studies demonstrate that while the loop 3 helix is a key functional domain, its function does not exclusively rely on any single residue.

The structure of lactoferrin-binding protein B from Neisseria meningitidis suggests roles in iron acquisition and neutralization of host defences

Pathogens have evolved a range of mechanisms to acquire iron from the host during infection. Several Gram-negative pathogens including members of the genera Neisseria and Moraxella have evolved two-component systems that can extract iron from the host glycoproteins lactoferrin and transferrin. The homologous iron-transport systems consist of a membrane-bound transporter and an accessory lipoprotein. While the mechanism behind iron acquisition from transferrin is well understood, relatively little is known regarding how iron is extracted from lactoferrin. Here, the crystal structure of the N-terminal domain (N-lobe) of the accessory lipoprotein lactoferrin-binding protein B (LbpB) from the pathogen Neisseria meningitidis is reported. The structure is highly homologous to the previously determined structures of the accessory lipoprotein transferrin-binding protein B (TbpB) and LbpB from the bovine pathogen Moraxella bovis. Docking the LbpB structure with lactoferrin reveals extensive binding interactions with the N1 subdomain of lactoferrin. The nature of the interaction precludes apolactoferrin from binding LbpB, ensuring the specificity of iron-loaded lactoferrin. The specificity of LbpB safeguards proper delivery of iron-bound lactoferrin to the transporter lactoferrin-binding protein A (LbpA). The structure also reveals a possible secondary role for LbpB in protecting the bacteria from host defences. Following proteolytic digestion of lactoferrin, a cationic peptide derived from the N-terminus is released. This peptide, called lactoferricin, exhibits potent antimicrobial effects. The docked model of LbpB with lactoferrin reveals that LbpB interacts extensively with the N-terminal lactoferricin region. This may provide a venue for preventing the production of the peptide by proteolysis, or directly sequestering the peptide, protecting the bacteria from the toxic effects of lactoferricin.

Iron- and hemin-dependent gene expression of Porphyromonas gingivalis

Although iron under anaerobic conditions is more accessible and highly reactive because of its reduced form, iron-dependent regulation is not well known in anaerobic bacteria. Here, we investigated iron- and hemin-dependent gene regulation in Porphyromonas gingivalis, an established periodontopathogen that primarily inhabits anaerobic pockets. Whole-genome microarrays of P. gingivalis genes were used to compare the levels of gene expression under iron-replete and iron-depleted conditions as well as under hemin-replete and hemin-depleted conditions. Under iron-depleted conditions, the expression of genes encoding proteins that participate in iron uptake and adhesion/invasion of host cells was increased, while that of genes encoding proteins involved in iron storage, energy metabolism, and electron transport was decreased. Interestingly, many of the genes with altered expression had no known function. Limiting the amount of hemin also resulted in a reduced expression of the genes encoding proteins involved in energy metabolism and electron transport. However, hemin also had a significant effect on many other biological processes such as oxidative stress protection and lipopolysaccharide synthesis. Overall, comparison of the data from iron-depleted conditions to those from hemin-depleted ones showed that although some regulation is through the iron derived from hemin, there also is significant distinct regulation through hemin only. Furthermore, our data showed that the molecular mechanisms of iron-dependent regulation are novel as the deletion of the putative Fur protein had no effect on the expression of iron-regulated genes. Finally, our functional studies demonstrated greater survivability of host cells in the presence of the iron-stressed bacterium than the iron-replete P. gingivalis cells. The major iron-regulated proteins encoded by PG1019-20 may play a role in this process as deletion of these sequences also resulted in reduced survival of the bacterium when grown with eukaryotic cells. Taken together, the results of this study demonstrated the utility of whole-genome microarray analysis for the identification of genes with altered expression profiles during varying growth conditions and provided a framework for the detailed analysis of the molecular mechanisms of iron and hemin acquisition, metabolism and virulence of P. gingivalis.

Identification of regulatory elements that control expression of the tbpBA operon in Neisseria gonorrhoeae

Iron is an essential nutrient for survival and establishment of infection by Neisseria gonorrhoeae. The neisserial transferrin binding proteins (Tbps) comprise a bipartite system for iron acquisition from human transferrin. TbpA is the TonB-dependent transporter that accomplishes iron internalization. TbpB is a surface-exposed lipoprotein that makes the iron uptake process more efficient. Previous studies have shown that the genes encoding these proteins are arranged in a bicistronic operon, with the tbpB gene located upstream of tbpA and separated from it by an inverted repeat. The operon is under the control of the ferric uptake regulator (Fur); however, promoter elements necessary for regulated expression of the genes have not been experimentally defined. In this study, putative regulatory motifs were identified and confirmed by mutagenesis. Further examination of the sequence upstream of these promoter/operator motifs led to the identification of several novel repeats. We hypothesized that these repeats are involved in additional regulation of the operon. Insertional mutagenesis of regions upstream of the characterized promoter region resulted in decreased tbpB and tbpA transcript levels but increased protein levels for both TbpA and TbpB. Using RNA sequencing (RNA-Seq) technology, we determined that a long RNA was produced from the region upstream of tbpB. We localized the 5' endpoint of this transcript to between the two upstream insertions by qualitative RT-PCR. We propose that expression of this upstream RNA leads to optimized expression of the gene products from within the tbpBA operon.

Identification of a Haemophilus influenzae factor H-Binding lipoprotein involved in serum resistance

Haemophilus influenzae is a Gram-negative human pathogen that resides in the upper respiratory tract. Encapsulated H. influenzae type b (Hib) and type f (Hif) are the most common serotypes associated with invasive disease. H. influenzae displays various strategies to circumvent the host innate immune response, including the bactericidal effect of the complement system. In this study, we identified an H. influenzae lipoprotein having the ability to bind factor H (FH), the major regulator of the alternative pathway of complement activation. This protein, named protein H (PH), was surface exposed and was found in all clinical Hib and Hif isolates tested. Deletion of the gene encoding for PH (lph) in Hib and Hif significantly reduced the interaction between bacteria and FH. When Hib and Hif PH variants were separately expressed in nontypeable (unencapsulated) H. influenzae, which did not bind FH, an increased FH affinity was observed. We recombinantly expressed the two PH variants in Escherichia coli, and despite sharing only 56% identical amino acids, both FH-binding Haemophilus proteins similarly interacted with the complement regulator FH short consensus repeats 7 and 18-20. Importantly, Hib and Hif resistance against the bactericidal effect of human serum was significantly reduced when bacterial mutants devoid of PH were tested. In conclusion, we have characterized a hitherto unknown bacterial protein that is crucial for mediating an interaction between the human pathogen H. influenzae and FH. This novel interaction is important for H. influenzae resistance against complement activation and will consequently promote bacterial pathogenesis.

Shared and distinct mechanisms of iron acquisition by bacterial and fungal pathogens of humans

Iron is the most abundant transition metal in the human body and its bioavailability is stringently controlled. In particular, iron is tightly bound to host proteins such as transferrin to maintain homeostasis, to limit potential damage caused by iron toxicity under physiological conditions and to restrict access by pathogens. Therefore, iron acquisition during infection of a human host is a challenge that must be surmounted by every successful pathogenic microorganism. Iron is essential for bacterial and fungal physiological processes such as DNA replication, transcription, metabolism, and energy generation via respiration. Hence, pathogenic bacteria and fungi have developed sophisticated strategies to gain access to iron from host sources. Indeed, siderophore production and transport, iron acquisition from heme and host iron-containing proteins such as hemoglobin and transferrin, and reduction of ferric to ferrous iron with subsequent transport are all strategies found in bacterial and fungal pathogens of humans. This review focuses on a comparison of these strategies between bacterial and fungal pathogens in the context of virulence and the iron limitation that occurs in the human body as a mechanism of innate nutritional defense.

Conserved regions of gonococcal TbpB are critical for surface exposure and transferrin iron utilization

The transferrin-binding proteins TbpA and TbpB enable Neisseria gonorrhoeae to obtain iron from human transferrin. The lipoprotein TbpB facilitates, but is not strictly required for, TbpA-mediated iron acquisition. The goal of the current study was to determine the contribution of two conserved regions within TbpB to the function of this protein. Using site-directed mutagenesis, the first mutation we constructed replaced the lipobox (LSAC) of TbpB with a signal I peptidase cleavage site (LAAA), while the second mutation deleted a conserved stretch of glycine residues immediately downstream of the lipobox. We then evaluated the resulting mutants for effects on TbpB expression, surface exposure, and transferrin iron utilization. Western blot analysis and palmitate labeling indicated that the lipobox, but not the glycine-rich motif, is required for lipidation of TbpB and tethering to the outer membrane. TbpB was released into the supernatant by the mutant that produces TbpB LSAC. Neither mutation disrupted the transport of TbpB across the bacterial cell envelope. When these mutant TbpB proteins were produced in a strain expressing a form of TbpA that requires TbpB for iron acquisition, growth on transferrin was either abrogated or dramatically diminished. We conclude that surface tethering of TbpB is required for optimal performance of the transferrin iron acquisition system, while the presence of the polyglycine stretch near the amino terminus of TbpB contributes significantly to transferrin iron transport function. Overall, these results provide important insights into the functional roles of two conserved motifs of TbpB, enhancing our understanding of this critical iron uptake system.

The transferrin-iron import system from pathogenic Neisseria species

Two pathogenic species within the genus Neisseria cause the diseases gonorrhoea and meningitis. While vaccines are available to protect against four N. meningitidis serogroups, there is currently no commercial vaccine to protect against serogroup B or against N. gonorrhoeae. Moreover, the available vaccines have significant limitations and with antibiotic resistance becoming an alarming issue, the search for effective vaccine targets to elicit long-lasting protection against Neisseria species is becoming more urgent. One strategy for vaccine development has targeted the neisserial iron import systems. Without iron, the Neisseriae cannot survive and, therefore, these iron import systems tend to be relatively well conserved and are promising vaccine targets, having the potential to offer broad protection against both gonococcal and meningococcal infections. These efforts have been boosted by recent reports of the crystal structures of the neisserial receptor proteins TbpA and TbpB, each solved in complex with human transferrin, an iron binding protein normally responsible for delivering iron to human cells. Here, we review the recent structural reports and put them into perspective with available functional studies in order to derive the mechanism(s) for how the pathogenic Neisseriae are able to hijack human iron transport systems for their own survival and pathogenesis.

Steric and allosteric factors prevent simultaneous binding of transferrin-binding proteins A and B to transferrin

The ability to acquire iron directly from host Tf (transferrin) is an adaptation common to important bacterial pathogens belonging to the Pasteurellaceae, Moraxellaceae and Neisseriaceae families. A surface receptor comprising an integral outer membrane protein, TbpA (Tf-binding protein A), and a surface-exposed lipoprotein, TbpB (Tf-binding protein B), mediates the iron acquisition process. TbpB is thought to extend from the cell surface for capture of Tf to initiate the process and deliver Tf to TbpA. TbpA functions as a gated channel for the passage of iron into the periplasm. In the present study we have mapped the effect of TbpA from Actinobacillus pleuropneumoniae on pTf (porcine Tf) using H/DX-MS (hydrogen/deuterium exchange coupled to MS) and compare it with a previously determined binding site for TbpB. The proposed TbpA footprint is adjacent to and potentially overlapping the TbpB-binding site, and induces a structural instability in the TbpB site. This suggests that simultaneous binding to pTf by both receptors would be hindered. We demonstrate that a recombinant TbpB lacking a portion of its anchor peptide is unable to form a stable ternary TbpA–pTf–TbpB complex. This truncated TbpB does not bind to a preformed Tf–TbpA complex, and TbpA removes pTf from a preformed Tf–TbpB complex. Thus the results of the present study support a model whereby TbpB ‘hands-off’ pTf to TbpA, which completes the iron removal and transport process.

Evidence of Fe3+ interaction with the plug domain of the outer membrane transferrin receptor protein of Neisseria gonorrhoeae: implications for Fe transport

Neisseria gonorrhoeae is an obligate pathogen that hijacks iron from the human iron transport protein, holo-transferrin (Fe(2)-Tf), by expressing TonB-dependent outer membrane receptor proteins, TbpA and TbpB. Homologous to other TonB-dependent outer membrane transporters, TbpA is thought to consist of a β-barrel with an N-terminal plug domain. Previous reports by our laboratories show that the sequence EIEYE in the plug domain is highly conserved among various bacterial species that express TbpA and plays a crucial role in iron utilization for gonococci. We hypothesize that this highly conserved EIEYE sequence in the TbpA plug, rich in hard oxygen donor groups, binds with Fe(3+) through the transport process across the outer membrane through the β-barrel. Sequestration of Fe(3+) by the TbpA-plug supports the paradigm that the ferric iron must always remain chelated and controlled throughout the transport process. In order to test this hypothesis here we describe the ability of both the recombinant wild-type plug, and three small peptides that encompass the sequence EIEYE of the plug, to bind Fe(3+). This is the first report of the expression/isolation of the recombinant wild-type TbpA plug. Although CD and SUPREX spectroscopies suggest that a non-native structure is observed for the recombinant plug, fluorescence quenching titrations indicate that the wild-type recombinant TbpA plug binds Fe (3+) with a conditional log K(d) = 7 at pH 7.5, with no evidence of binding at pH 6.3. A recombinant TbpA plug with mutated sequence (NEIEYEN → NEIAAAN) shows no evidence of Fe(3+) binding under our experimental set up. Interestingly, in silico modeling with the wild-type plug also predicts a flexible loop structure for the EIEYE sequence under native conditions which once again supports the Fe(3+) binding hypothesis. These in vitro observations are consistent with the hypothesis that the EIEYE sequence in the wild-type TbpA plug binds Fe(3+) during the outer membrane transport process in vivo.

Anchor peptide of transferrin-binding protein B is required for interaction with transferrin-binding protein A

Gram-negative bacterial pathogens belonging to the Pasteurellaceae, Moraxellaceae, and Neisseriaceae families rely on an iron acquisition system that acquires iron directly from host transferrin (Tf). The process is mediated by a surface receptor composed of transferrin-binding proteins A and B (TbpA and TbpB). TbpA is an integral outer membrane protein that functions as a gated channel for the passage of iron into the periplasm. TbpB is a surface-exposed lipoprotein that facilitates the iron uptake process. In this study, we demonstrate that the region encompassing amino acids 7-40 of Actinobacillus pleuropneumoniae TbpB is required for forming a complex with TbpA and that the formation of the complex requires the presence of porcine Tf. These results are consistent with a model in which TbpB is responsible for the initial capture of iron-loaded Tf and subsequently interacts with TbpA through the anchor peptide. We propose that TonB binding to TbpA initiates the formation of the TbpB-TbpA complex and transfer of Tf to TbpA.

FbpA--a bacterial transferrin with more to offer

BACKGROUND

Gram negative bacteria require iron for growth and virulence. It has been shown that certain pathogenic bacteria such as Neisseria gonorrhoeae possess a periplasmic protein called ferric binding protein (FbpA), which is a node in the transport of iron from the cell exterior to the cytosol.

SCOPE OF REVIEW

The relevant literature is reviewed which establishes the molecular mechanism of FbpA mediated iron transport across the periplasm to the inner membrane.

MAJOR CONCLUSIONS

Here we establish that FbpA may be considered a bacterial transferrin on structural and functional grounds. Data are presented which suggest a continuum whereby FbpA may be considered as a naked iron carrier, as well as a Fe-chelate carrier, and finally a member of the larger family of periplasmic binding proteins.

GENERAL SIGNIFICANCE

An investigation of the molecular mechanisms of action of FbpA as a member of the transferrin super family enhances our understanding of bacterial mechanisms for acquisition of the essential nutrient iron, as well as the modes of action of human transferrin, and may provide approaches to the control of pathogenic diseases. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

TonB-Dependent Transporters Expressed by Neisseria gonorrhoeae

Neisseria gonorrhoeae causes the common sexually transmitted infection, gonorrhea. This microorganism is an obligate human pathogen, existing nowhere in nature except in association with humans. For growth and proliferation, N. gonorrhoeae requires iron and must acquire this nutrient from within its host. The gonococcus is well-adapted for growth in diverse niches within the human body because it expresses efficient transport systems enabling use of a diverse array of iron sources. Iron transport systems facilitating the use of transferrin, lactoferrin, and hemoglobin have two components: one TonB-dependent transporter and one lipoprotein. A single component TonB-dependent transporter also allows N. gonorrhoeae to avail itself of iron bound to heterologous siderophores produced by bacteria within the same ecological niche. Other TonB-dependent transporters are encoded by the gonococcus but have not been ascribed specific functions. The best characterized iron transport system expressed by N. gonorrhoeae enables the use of human transferrin as a sole iron source. This review summarizes the molecular mechanisms involved in gonococcal iron acquisition from human transferrin and also reviews what is currently known about the other TonB-dependent transport systems. No vaccine is available to prevent gonococcal infections and our options for treating this disease are compromised by the emergence of antibiotic resistance. Because iron transport systems are critical for the survival of the gonococcus in vivo, the surface-exposed components of these systems are attractive candidates for vaccine development or therapeutic intervention.

The fbpABC operon is required for Ton-independent utilization of xenosiderophores by Neisseria gonorrhoeae strain FA19

Neisseria gonorrhoeae produces no known siderophores but can employ host-derived, iron-binding proteins, including transferrin and lactoferrin, as iron sources. Given the propensity of this pathogen to hijack rather than synthesize iron-sequestering molecules, we hypothesized that the ability to use siderophores produced by other bacteria, or xenosiderophores, may also play a role in the survival of the gonococcus. Among a panel of diverse siderophores, only the catecholate xenosiderophores enterobactin and salmochelin promoted growth of gonococcal strain FA19. Surprisingly, the internalization pathway was independent of TonB or any of the TonB-dependent transporters. Xenosiderophore-mediated growth was similarly independent of the pilin-extruding secretin formed by PilQ and of the hydrophobic-agent efflux system composed of MtrCDE. The fbpABC operon encodes a periplasmic-binding-protein-dependent ABC transport system that enables the gonococcus to transport iron into the cell subsequent to outer membrane translocation. We hypothesized that the FbpABC proteins, required for ferric iron transport from transferrin and lactoferrin, might also contribute to the utilization of xenosiderophores as iron sources. We created mutants that conditionally expressed FbpABC from an IPTG-inducible promoter. We determined that expression of FbpABC was required for growth of gonococcal strain FA19 in the presence of enterobactin and salmochelin. The monomeric component of enterobactin, dihydroxybenzoylserine (DHBS), and the S2 form of salmochelin specifically promoted FbpABC-dependent growth of FA19. This study demonstrated that the gonococcal FbpABC transport system is required for utilization of some xenosiderophores as iron sources and that growth promotion by these ferric siderophores can occur in the absence of TonB or individual TonB-dependent transporters.

Kinetic analysis of ligand interaction with the gonococcal transferrin-iron acquisition system

The transferrin iron acquisition system of Neisseria gonorrhoeae consists of two dissimilar transferrin binding proteins (Tbp) A and B. TbpA is a TonB dependent transporter while TbpB is a lipoprotein that makes iron acquisition from transferrin (Tf) more efficient. In an attempt to further define the individual roles of these receptors in the process of Tf-iron acquisition, the kinetics of the receptor proteins in regards to ligand association and dissociation were evaluated. Tf association with TbpB was rapid as compared to TbpA. Tf dissociation from the wild-type receptor occurred in a biphasic manner; an initial rapid release was followed by a slower dissociation over time. Both TbpA and TbpB demonstrated a two-phase release pattern; however, TbpA required both TonB and TbpB for efficient Tf dissociation from the cell surface. The roles of TbpA and TbpB in Tf dissociation were further examined, utilizing previously created HA fusion proteins. Using a Tf-utilization deficient TbpA-HA mutant, we concluded that the slower rate of ligand dissociation demonstrated by the wild-type transporter was a function of successful iron internalization. Insertion into the C-terminus of TbpB decreased the rate of Tf dissociation, while insertion into the N-terminus had no effect on this process. From these studies, we propose that TbpA and TbpB function synergistically during the process of Tf iron acquisition and that TbpB makes the process of Tf-iron acquisition more efficient at least in part by affecting association and dissociation of Tf from the cell surface.

Hijacking transferrin bound iron: protein-receptor interactions involved in iron transport in N. gonorrhoeae

Neisseria gonorrhoeae has the capacity to acquire iron from its human host by removing this essential nutrient from serum transferrin. The transferrin binding proteins, TbpA and TbpB constitute the outer membrane receptor complex responsible for binding transferrin, extracting the tightly bound iron from the host-derived molecule, and transporting iron into the periplasmic space of this Gram-negative bacterium. Once iron is transported across the outer membrane, ferric binding protein A (FbpA) moves the iron across the periplasmic space and initiates the process of transport into the bacterial cytosol. The results of the studies reported here define the multiple steps in the iron transport process in which TbpA and TbpB participate. Using the SUPREX technique for assessing the thermodynamic stability of protein-ligand complexes, we report herein the first direct measurement of periplasmic FbpA binding to the outer membrane protein TbpA. We also show that TbpA discriminates between apo- and holo-FbpA; i.e. the TbpA interaction with apo-FbpA is higher affinity than the TbpA interaction with holo-FbpA. Further, we demonstrate that both TbpA and TbpB individually can deferrate transferrin and ferrate FbpA without energy supplied from TonB resulting in sequestration by apo-FbpA.

Identification and characterization of gonococcal iron transport systems as potential vaccine antigens

Gonorrhea is the second most commonly reported infectious disease in the USA, and incidence has been increasing in recent years. Antibiotic resistance among clinical isolates has reached a critical point at which the CDC currently recommends only a single class of antibiotic for treatment. These developments have hastened the search for a vaccine to protect against gonococcal infections. Vaccine efforts have been thwarted by the ability of the gonococcus to antigenically vary most surface structures. The transferrin-iron transport system is not subject to high-frequency phase or antigenic variation and is expressed by all pathogenic Neisseria. Vaccine formulations comprised of epitopes of the transferrin-binding proteins complexed with inactivated cholera toxin generated antibodies with potentially protective characteristics. These antigens, and others predicted from genome sequence data, could be developed into a vaccine that protects against neisserial infections.

Identification of TbpA residues required for transferrin-iron utilization by Neisseria gonorrhoeae

Neisseria gonorrhoeae requires iron for survival in the human host and therefore expresses high-affinity receptors for iron acquisition from host iron-binding proteins. The gonococcal transferrin-iron uptake system is composed of two transferrin binding proteins, TbpA and TbpB. TbpA is a TonB-dependent, outer membrane transporter critical for iron acquisition, while TbpB is a surface-exposed lipoprotein that increases the efficiency of iron uptake. The precise mechanism by which TbpA mediates iron acquisition has not been elucidated; however, the process is distinct from those of characterized siderophore transporters. Similar to these TonB-dependent transporters, TbpA is proposed to have two distinct domains, a beta-barrel and a plug domain. We hypothesize that the TbpA plug coordinates iron and therefore potentially functions in multiple steps of transferrin-mediated iron acquisition. To test this hypothesis, we targeted a conserved motif within the TbpA plug domain and generated single, double, and triple alanine substitution mutants. Mutagenized TbpAs were expressed on the gonococcal cell surface and maintained wild-type transferrin binding affinity. Single alanine substitution mutants internalized iron at wild-type levels, while the double and triple mutants showed a significant decrease in iron uptake. Moreover, the triple alanine substitution mutant was unable to grow on transferrin as a sole iron source; however, expression of TbpB compensated for this defect. These data indicate that the conserved motif between residues 120 and 122 of the TbpA plug domain is critical for transferrin-iron utilization, suggesting that this region plays a role in iron acquisition that is shared by both TbpA and TbpB.

Identification of transferrin-binding domains in TbpB expressed by Neisseria gonorrhoeae

The transferrin iron acquisition system of Neisseria gonorrhoeae is necessary for iron uptake from transferrin in the human host and requires the participation of two distinct proteins: TbpA and TbpB. TbpA is a TonB-dependent outer membrane transporter responsible for the transport of iron into the cell. TbpB is a lipid-modified protein, for which a precise role in receptor function has not yet been elucidated. These receptor complex proteins show promise as vaccine candidates; therefore, it is important to identify surface-exposed regions of the proteins required for wild-type functions. In this study we examined TbpB, which has been reported to be surface exposed in its entirety; however, this hypothesis has never been tested experimentally. We placed the hemagglutinin (HA) epitope into TbpB with the dual purpose of examining the surface exposure of particular epitopes as well as their impact on receptor function. Nine insertion mutants were created, placing the epitope downstream of the signal peptidase II cleavage site. We report that the HA epitope is surface accessible in all mutants, indicating that the full-length TbpB is completely surface exposed. By expressing the TbpB-HA fusion proteins in N. gonorrhoeae, we were able to examine the impact of each insertion on the function of TbpB and the transferrin acquisition process. We propose that TbpB is comprised of two transferrin-binding-competent lobes, both of which are critical for efficient iron uptake from human transferrin.

The pilus and porin of Neisseria gonorrhoeae cooperatively induce Ca2+ transients in infected epithelial cells

Purified pili and porin from Neisseria quickly mobilize calcium (Ca(2+)) stores in monocytes and epithelial cells, ultimately influencing host cell viability as well as bacterial intracellular survival. Here, we examined the Ca(2+) transients induced in human epithelial cells during infection by live, piliated N. gonorrhoeae. Porin induced an influx of Ca(2+) from the extracellular medium less than 60 s post infection. The porin-induced transient is followed by a pilus-induced release of Ca(2+) from intracellular stores. The timing of these events is similar to that observed using purified proteins. Interestingly, the porin-induced Ca(2+) flux is required for the pilus-induced transient, indicating that the pilus-induced Ca(2+) release is, itself, Ca(2+) dependent. Several lines of evidence indicate that porin is present on pili. Moreover, pilus retraction strongly influences the porin- and pilus-induced Ca(2+) fluxes. These and other results strongly suggest that the pilus and porin cooperate to modulate calcium signalling in epithelial cells, and propose a model to explain how N. gonorrhoeae triggers Ca(2+) transients in the initial stages of pilus-mediated attachment.

Solenopsis invicta transferrin: cDNA cloning, gene architecture, and up-regulation in response to Beauveria bassiana infection

Transferrin genes from several insects have been shown to be induced in response to bacterial or fungal infection. We were interested to know whether transferrin genes in the red imported fire ant, Solenopsis invicta, are similarly induced by microbial challenge. Hence, the cDNA and structure of a gene exhibiting significant homology to insect transferrins were elucidated for S. invicta. The cDNA was comprised of 2417 nucleotides, excluding the poly(A) tail, with a large open reading frame of 2106 nucleotides. The predicted translation product of the S. invicta tranferrin (SiTf) gene was a 702 amino acid polypeptide with an estimated molecular mass of 77.3 kDa and a pI value of 5.66, characteristics consistent with transferrin proteins. Comparative analysis of genomic and cDNA sequences revealed that the SiTf gene was comprised of 8 exons. Quantitative real-time PCR was used to examine the expression of SiTf. Expression of SiTf was induced in worker ants exposed to Beauveria bassiana conidia. Autoclave-killed conidia did not elicit a SiTf induction response from worker ants. Genes, like SiTf, responding to microbe attack or infection may provide a unique approach to assist in the discovery of microbial control organisms for the target insect pest.

Cloning and expression of genes encoding transferrin-binding protein A and B from Actinobacillus pleuropneumoniae serotype 5

Actinobacillus pleuropneumoniae is an important primary pathogen in pigs, which causes a highly contagious pleuropneumonia. As an adaptation to the iron-restricted environment of the host, A. pleuropneumoniae possesses iron acquisition pathways mediated by surface receptors that specifically bind transferrin from the host. The receptor is composed of two receptor proteins, transferrin-binding protein A and B (TbpA and B), which are both capable of binding to transferrin. An impairment of iron uptake mechanisms is likely to reduce virulence. For this reason, these two proteins can be useful as a candidate target for A. pleuropneumoniae vaccination. To do this, genes encoding the TbpA and B from a serotype 5 isolate of A. pleuropneumoniae were amplified from genomic DNA template by PCR and cloned into a pRSET prokaryotic expression vector, generating the pRSET-A.pp-TbpA and B. Escherichia coli BL21(DE3)pLysS competent cells were transformed with each construct followed by the induction of protein expression by the addition of IPTG. Bands corresponding to the predicted sizes (110 and 60 kDa) were seen on the SDS-PAGE. Polyclonal antibodies raised against recombinant TbpA and B from mice were reacted with bacterial proteins. This result indicates that the recombinant proteins can induce immunological responses and might be useful as candidate targets for A. pleuropneumoniae vaccination.

Iron and heme utilization in Porphyromonas gingivalis

Porphyromonas gingivalis is a Gram-negative anaerobic bacterium associated with the initiation and progression of adult periodontal disease. Iron is utilized by this pathogen in the form of heme and has been shown to play an essential role in its growth and virulence. Recently, considerable attention has been given to the characterization of various secreted and surface-associated proteins of P. gingivalis and their contribution to virulence. In particular, the properties of proteins involved in the uptake of iron and heme have been extensively studied. Unlike other Gram-negative bacteria, P. gingivalis does not produce siderophores. Instead it employs specific outer membrane receptors, proteases (particularly gingipains), and lipoproteins to acquire iron/heme. In this review, we will focus on the diverse mechanisms of iron and heme acquisition in P. gingivalis. Specific proteins involved in iron and heme capture will be described. In addition, we will discuss new genes for iron/heme utilization identified by nucleotide sequencing of the P. gingivalis W83 genome. Putative iron- and heme-responsive gene regulation in P. gingivalis will be discussed. We will also examine the significance of heme/hemoglobin acquisition for the virulence of this pathogen.

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.

Bacterial iron sources: from siderophores to hemophores

Iron is an essential element for most organisms, including bacteria. The oxidized form is insoluble, and the reduced form is highly toxic for most macromolecules and, in biological systems, is generally sequestrated by iron- and heme-carrier proteins. Thus, despite its abundance on earth, there is practically no free iron available for bacteria whatever biotope they colonize. To fulfill their iron needs, bacteria have multiple iron acquisition systems, reflecting the diversity of their potential biotopes. The iron/heme acquisition systems in bacteria have one of two general mechanisms. The first involves direct contact between the bacterium and the exogenous iron/heme sources. The second mechanism relies on molecules (siderophores and hemophores) synthesized and released by bacteria into the extracellular medium; these molecules scavenge iron or heme from various sources. Recent genetic, biochemical, and crystallographic studies have allowed substantial progress in describing molecular mechanisms of siderophore and hemophore interactions with the outer membrane receptors, transport through the inner membrane, iron storage, and regulation of genes encoding biosynthesis and uptake proteins.

The bacterial receptor protein, transferrin-binding protein B, does not independently facilitate the release of metal ion from human transferrin

Pathogenic Gram-negative bacteria of the Pasteurellaceae and Neisseriaceae acquire iron for growth from host transferrin through the action of specific surface receptors. Iron is removed from transferrin by the receptor at the cell surface and is transported across the outer membrane to the periplasm. A periplasmic binding protein-dependent pathway subsequently transports iron into the cell. The transferrin receptor is composed of a largely surface-exposed lipoprotein, transferrin binding protein B, and a TonB-dependent integral outer membrane protein, transferrin binding protein A. To examine the role of transferrin binding protein B in the iron removal process, complexes of recombinant transferrin binding protein B and transferrin were prepared and compared with transferrin in metal-binding and -removal experiments. A polyhistidine-tagged form of recombinant transferrin binding protein B was able to purify a complex with transferrin that was largely monodisperse by dynamic light scattering analysis. Gallium was used instead of iron in the metal-binding studies, since it resulted in increased stability of recombinant transferrin binding protein B in the complex. Difference absorption spectra were used to monitor removal of gallium by nitrilotriacetic acid. Kinetic and equilibrium binding studies indicated that transferrin binds gallium more tightly in the presence of transferrin binding protein B. Thus, transferrin binding protein B does not facilitate metal ion removal and additional components are required for this process.

The plug domain of a neisserial TonB-dependent transporter retains structural integrity in the absence of its transmembrane beta-barrel

Transferrin binding protein A (TbpA) is a TonB-dependent outer membrane protein expressed by pathogenic bacteria for iron acquisition from human transferrin. The N-terminal 160 residues (plug domain) of TbpA were overexpressed in both the periplasm and cytoplasm of Escherichia coli. We found this domain to be soluble and monodisperse in solution, exhibiting secondary structure elements found in plug domains of structurally characterized TonB-dependent transporters. Although the TbpA plug domain is apparently correctly folded, we were not able to observe an interaction with human transferrin by isothermal titration calorimetry or nitrocellulose binding assays. These experiments suggest that the plug domain may fold independently of the beta-barrel, but extracellular loops of the beta-barrel are required for ligand binding.

Role of transferrin receptor from a Neisseria meningitidis tbpB isotype II strain in human transferrin binding and virulence

Neisseria meningitidis acquires iron through the action of the transferrin (Tf) receptor, which is composed of the Tf-binding proteins A and B (TbpA and TbpB). Meningococci can be classified into isotype I and II strains depending on whether they harbor a type I or II form of TbpB. Both types of TbpB have been shown to differ in their genomic, biochemical, and antigenic properties. Here we present a comparative study of isogenic mutants deficient in either or both Tbps from the isotype I strain B16B6 and isotype II strain M982. We show that TbpA is essential in both strains for iron uptake and growth with iron-loaded human Tf as a sole iron source. No growth has also been observed for the TbpB- mutant of strain B16B6, as shown previously, whereas the growth of the analogous mutant in M982 was similar to that in the wild type. This indicates that TbpB in the latter strain plays a facilitating but not essential role in iron uptake, which has been observed previously in similar studies of other bacteria. These data are discussed in relation to the fact that isotype II strains represent more than 80% of serogroup B meningococcal strains. The contribution of both subunits in the bacterial virulence of strain M982 has been assessed in a murine model of bacteremia. Both the TbpB- TbpA- mutant and the TbpA- mutant are shown to be nonvirulent in mice, whereas the virulence of the TbpB- mutant is similar to that of the wild type.

Analysis of haptoglobin and hemoglobin-haptoglobin interactions with the Neisseria meningitidis TonB-dependent receptor HpuAB by flow cytometry

Neisseria meningitidis expresses a two-component TonB-dependent receptor, HpuAB, which mediates heme-iron (Hm-Fe) acquisition from hemoglobin and hemoglobin-haptoglobin complexes. Due to genetic polymorphisms in the human haptoglobin gene, haptoglobin (and hemoglobin-haptoglobin) exists as three structurally distinct phenotypes. In this study, we examined the influence of the haptoglobin phenotype on the interactions of HpuAB with apo-haptoglobin and hemoglobin-haptoglobin. Growth assays confirmed that HpuAB utilizes hemoglobin-haptoglobin more efficiently than hemoglobin as an Fe source and revealed a preference for human-specific, polymeric 2-2 or 2-1 hemoglobin-haptoglobin complexes. We developed a flow cytometry-based assay to measure the binding kinetics of fluorescein-labeled ligands to HpuAB on live, intact meningococci. The binding affinity of HpuAB for hemoglobin-haptoglobin phenotypes correlated well with the ability of each ligand to support Neisseria meningitidis growth, with higher affinities exhibited for types 2-2 and 2-1 hemoglobin-haptoglobin. Saturable binding of Hb and apo-haptoglobin suggested that HpuAB-mediated utilization of hemoglobin-haptoglobin involves specific interactions with both components. In contrast to previous studies, we detected binding of HpuB expressed alone to hemoglobin, apo-haptoglobin, and hemoglobin-haptoglobin of all three phenotypes. However, in the absence of HpuA, the binding capacity and/or affinity of the receptor was reduced and the dissociation of hemoglobin was impaired. We did not detect binding of HpuA alone to hemoglobin, apo-haptoglobin, or hemoglobin-haptoglobin; however, the lipoprotein is crucial for optimal recognition and use of ligands by the receptor. Finally, this study confirmed the integral role of TonB and the proton motive force in the binding and dissociation of Hb and hemoglobin-haptoglobin from HpuAB.

Determination of surface-exposed, functional domains of gonococcal transferrin-binding protein A

The gonococcal transferrin receptor is composed of two distinct proteins, TbpA and TbpB. TbpA is a member of the TonB-dependent family of integral outer membrane transporters, while TbpB is lipid modified and thought to be peripherally surface exposed. We previously proposed a hypothetical topology model for gonococcal TbpA that was based upon computer predictions and similarity with other TonB-dependent transporters for which crystal structures have been determined. In the present study, the hemagglutinin epitope was inserted into TbpA to probe the surface topology of this protein and secondarily to test the functional impacts of site-specific mutagenesis. Twelve epitope insertion mutants were constructed, five of which allowed us to confirm the surface exposure of loops 2, 3, 5, 7, and 10. In contrast to the predictions set forth by the hypothetical model, insertion into the plug region resulted in an epitope that was surface accessible, while epitope insertions into two putative loops (9 and 11) were not surface accessible. Insertions into putative loop 3 and beta strand 9 abolished transferrin binding and utilization, and the plug insertion mutant exhibited decreased transferrin-binding affinity concomitant with an inability to utilize it. Insertion into putative beta strand 16 generated a mutant that was able to bind transferrin normally but that was unable to mediate utilization. Mutants with insertions into putative loops 2, 9, and 11 maintained wild-type binding affinity but could utilize only transferrin in the presence of TbpB. This is the first demonstration of the ability of TbpB to compensate for a mutation in TbpA.

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.

Insight into the structure and function of the transferrin receptor from Neisseria meningitidis using microcalorimetric techniques

The transferrin receptor of Neisseria meningitidis is composed of the transmembrane protein TbpA and the outer membrane protein TbpB. Both receptor proteins have the capacity to independently bind their ligand human transferrin (htf). To elucidate the specific role of these proteins in receptor function, isothermal titration calorimetry was used to study the interaction between purified TbpA, TbpB or the entire receptor (TbpA + TbpB) with holo- and apo-htf. The entire receptor was shown to contain a single high affinity htf-binding site on TbpA and approximately two lower affinity binding sites on TbpB. The binding sites appear to be independent. Purified TbpA was shown to have strong ligand preference for apo-htf, whereas TbpA in the receptor complex with TbpB preferentially binds the holo form of htf. The orientation of the ligand specificity of TbpA toward holo-htf is proposed to be the physiological function of TbpB. Furthermore, the thermodynamic mode of htf binding by TbpB of isotypes I and II was shown to be different. A protocol for the generation of active, histidine-tagged TbpB as well as its individual N- and C-terminal domains is presented. Both domains are shown to strongly interact with each other, and isothermal titration calorimetry and circular dichroism experiments provide clear evidence for this interaction causing conformational changes. The N-terminal domain of TbpB was shown to be the site of htf binding, whereas the C-terminal domain is not involved in binding. Furthermore, the interactions between TbpA and the different domains of TbpB have been demonstrated.

Identification of ZipA, a signal recognition particle-dependent protein from Neisseria gonorrhoeae

A genetic screen designed to identify proteins that utilize the signal recognition particle (SRP) for targeting in Escherichia coli was used to screen a Neisseria gonorrhoeae plasmid library. Six plasmids were identified in this screen, and each is predicted to encode one or more putative cytoplasmic membrane (CM) proteins. One of these, pSLO7, has three open reading frames (ORFs), two of which have no similarity to known proteins in GenBank other than sequences from the closely related N. meningitidis. Further analyses showed that one of these, SLO7ORF3, encodes a protein that is dependent on the SRP for localization. This gene also appears to be essential in N. gonorrhoeae since it was not possible to generate null mutations in the gene. Although appearing unique to Neisseria at the DNA sequence level, SLO7ORF3 was found to share some features with the cell division gene zipA of E. coli. These features included similar chromosomal locations (with respect to linked genes) as well as similarities in the predicted protein domain structures. Here, we show that SLO7ORF3 can complement an E. coli conditional zipA mutant and therefore encodes a functional ZipA homolog in N. gonorrhoeae. This observation is significant in that it is the first ZipA homolog identified in a non-rod-shaped organism. Also interesting is that this is the fourth cell division protein (the others are FtsE, FtsX, and FtsQ) shown to utilize the SRP for localization, which may in part explain why the genes encoding the three SRP components are essential in bacteria.