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

The milk-derived lactoferrin inhibits V-ATPase activity by targeting its V1 domain

Lactoferrin (Lf), a bioactive milk protein, exhibits strong anticancer and antifungal activities. The search for Lf targets and mechanisms of action is of utmost importance to enhance its effective applications. A common feature among Lf-treated cancer and fungal cells is the inhibition of a proton pump called V-ATPase. Lf-driven V-ATPase inhibition leads to cytosolic acidification, ultimately causing cell death of cancer and fungal cells. Given that a detailed elucidation of how Lf and V-ATPase interact is still missing, herein we aimed to fill this gap by employing a five-stage computational approach. Molecular dynamics simulations of both proteins were performed to obtain a robust sampling of their conformational landscape, followed by clustering, which allowed retrieving representative structures, to then perform protein-protein docking. Subsequently, molecular dynamics simulations of the docked complexes and free binding energy calculations were carried out to evaluate the dynamic binding process and build a final ranking based on the binding affinities. Detailed atomist analysis of the top ranked complexes clearly indicates that Lf binds to the V₁ cytosolic domain of V-ATPase. Particularly, our data suggest that Lf binds to the interfaces between A/B subunits, where the ATP hydrolysis occurs, thus inhibiting this process. The free energy decomposition analysis further identified key binding residues that will certainly aid in the rational design of follow-up experimental studies, hence bridging computational and experimental biochemistry.

Lactoferrin binding protein B - a bi-functional bacterial receptor protein

Lactoferrin binding protein B (LbpB) is a bi-lobed outer membrane-bound lipoprotein that comprises part of the lactoferrin (Lf) receptor complex in Neisseria meningitidis and other Gram-negative pathogens. Recent studies have demonstrated that LbpB plays a role in protecting the bacteria from cationic antimicrobial peptides due to large regions rich in anionic residues in the C-terminal lobe. Relative to its homolog, transferrin-binding protein B (TbpB), there currently is little evidence for its role in iron acquisition and relatively little structural and biophysical information on its interaction with Lf. In this study, a combination of crosslinking and deuterium exchange coupled to mass spectrometry, information-driven computational docking, bio-layer interferometry, and site-directed mutagenesis was used to probe LbpB:hLf complexes. The formation of a 1:1 complex of iron-loaded Lf and LbpB involves an interaction between the Lf C-lobe and LbpB N-lobe, comparable to TbpB, consistent with a potential role in iron acquisition. The Lf N-lobe is also capable of binding to negatively charged regions of the LbpB C-lobe and possibly other sites such that a variety of higher order complexes are formed. Our results are consistent with LbpB serving dual roles focused primarily on iron acquisition when exposed to limited levels of iron-loaded Lf on the mucosal surface and effectively binding apo Lf when exposed to high levels at sites of inflammation.

A comparative, cross-species investigation of the properties and roles of transferrin- and lactoferrin-binding protein B from pathogenic bacteria

Pathogenic bacteria from the families Neisseriaeceae and Moraxellaceae acquire iron from their host using surface receptors that have the ability to hijack iron from the iron-sequestering host proteins transferrin (Tf) and lactoferrin (Lf). The process of acquiring iron from Tf has been well-characterized, including the role of the surface lipoprotein transferrin-binding protein B (TbpB). In contrast, the only well-defined role for the homologue, LbpB, is in its protection against cationic antimicrobial peptides, which is mediated by regions present in some LbpBs that are highly enriched in glutamic or aspartic acid. In this study we compare the Tf-TbpB and the Lf-LbpB interactions and examine the protective effect of LbpB against extracts from human and transgenic mouse neutrophils to gains insights into the physiological roles of LbpB. The results indicate that in contrast to the Tf-TbpB interaction, Lf-LbpB interaction is sensitive to pH and varies between species. In addition, the results with transgenic mouse neutrophils raise the question of whether there is species specificity in the cleavage of Lf to generate cationic antimicrobial peptides or differences in the potency of peptides derived from mouse and human Lf.

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.

Bacterial receptors for host transferrin and lactoferrin: molecular mechanisms and role in host–microbe interactions

Iron homeostasis in the mammalian host limits the availability of iron to invading pathogens and is thought to restrict iron availability for microbes inhabiting mucosal surfaces. The presence of surface receptors for the host iron-binding glycoproteins transferrin (Tf) and lactoferrin (Lf) in globally important Gram-negative bacterial pathogens of humans and food production animals suggests that Tf and Lf are important sources of iron in the upper respiratory or genitourinary tracts, where they exclusively reside. Lf receptors have the additional function of protecting against host cationic antimicrobial peptides, suggesting that the bacteria expressing these receptors reside in a niche where exposure is likely. In this review we compare Tf and Lf receptors with respect to their structural and functional features, their role in colonization and infection, and their distribution among pathogenic and commensal bacteria.

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.

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.

Bovine lactoferrin region responsible for binding to bifidobacterial cell surface proteins

Bovine lactoferrin promotes bifidobacterial growth. Its binding to bifidobacteria is thought to be responsible for such action. After separating the bovine lactoferrin half molecule and extraction of surface proteins from bifidobacteria, binding profiles were observed by immunoblotting. No binding appeared when lactoferrin C-lobe was reacted with the cell surface proteins on a polyvinylidene difluoride membrane. Conversely, a 50-kDa band appeared when the surface proteins were reacted with either intact or nicked bovine lactoferrin. This result strongly suggests that the binding region could be lactoferrin N-lobe. Interestingly, despite the absence of binding, C-lobe enhanced bifidobacterial growth.

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.

Structural Characterization of the Ceruloplasmin: Lactoferrin Complex in Solution

Ceruloplasmin is a copper protein found in vertebrate plasma, which belongs to the family of multicopper oxidases. Like transferrin of the blood plasma, lactoferrin, the iron-containing protein of human milk, saliva, tears, seminal plasma and of neutrophilic leukocytes tightly binds two ferric ions. Human lactoferrin and ceruloplasmin have been previously shown to interact both in vivo and in vitro forming a complex. Here we describe a study of the conformation of the human lactoferrin/ceruloplasmin complex in solution using small angle X-ray scattering. Our ab initio structural analysis shows that the complex has a 1:1 stoichiometry and suggests that complex formation occurs without major conformational rearrangements of either protein. Rigid-body modeling of the mutual arrangement of proteins in the complex essentially yields two families of solutions. Final discrimination is possible when integrating in the modeling process extra information translating into structural constraints on the interaction between the two partners.

High-affinity binding by the periplasmic iron-binding protein from Haemophilus influenzae is required for acquiring iron from transferrin

The periplasmic iron-binding protein, FbpA (ferric-ion-binding protein A), performs an essential role in iron acquisition from transferrin in Haemophilus influenzae. A series of site-directed mutants in the metal-binding amino acids of FbpA were prepared to determine their relative contribution to iron binding and transport. Structural studies demonstrated that the mutant proteins crystallized in an open conformation with the iron atom associated with the C-terminal domain. The iron-binding properties of the mutant proteins were assessed by several assays, including a novel competitive iron-binding assay. The relative ability of the proteins to compete for iron was pH dependent, with a rank order at pH 6.5 of wild-type, Q58L, H9Q>H9A, E57A>Y195A, Y196A. The genes encoding the mutant FbpA were introduced into H. influenzae and the resulting strains varied in the level of ferric citrate required to support growth on iron-limited medium, suggesting a rank order for metal-binding affinities under physiological conditions comparable with the competitive binding assay at pH 6.5 (wild-type=Q58L>H9Q>H9A, E57A>Y195A, Y196A). Growth dependence on human transferrin was only obtained with cells expressing wild-type, Q58L or H9Q FbpAs, proteins with stability constants derived from the competition assay >2.0x10(18) M(-1). These results suggest that a relatively high affinity of iron binding by FbpA is required for removal of iron from transferrin and its transport across the outer membrane.

Therapeutic potential of iron chelators in diseases associated with iron mismanagement

A considerable array of diseases are now recognized to be associated with misplacement of iron. Excessive deposits of the metal in sensitive tissue sites can result in formation of destructive hydroxyl radicals as well as in stimulation of growth of neoplastic and microbial cell invaders. To counteract potential iron damage, hosts employ the iron chelators, transferrin and lactoferrin. These proteins have been recently developed into pharmaceutical products. Additionally, a variety of low molecular mass iron chelators are being used/tested to treat whole body iron loading, and specific diseases for which the metal is a known or suspected risk factor.

Perspectives on interactions between lactoferrin and bacteriaThis paper is one of a selection of papers published in this Special Issue, entitled 7th International Conference on Lactoferrin: Structure, Function, and Applications, and has undergone the Journal's usual peer review process

Lactoferrin has long been recognized for its antimicrobial properties, initially attributed primarily to iron sequestration. It has since become apparent that interaction between the host and bacteria is modulated by a complex series of interactions between lactoferrin and bacteria, lactoferrin and bacterial products, and lactoferrin and host cells. The primary focus of this review is the interaction between lactoferrin and bacteria, but interactions with the lactoferrin-derived cationic peptide lactoferricin will also be discussed. We will summarize what is currently known about the interaction between lactoferrin (or lactoferricin) and surface or secreted bacterial components, comment on the potential physiological relevance of the findings, and identify key questions that remain unanswered.

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.