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

Abstract

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.

Lactoferrin in Human Milk of Prolonged Lactation

“…The iron-free form of Lf (apo-Lf) is a cause of iron deficiency in microorganisms as iron is required for life and growth rate [21]. Research has shown that the direct interaction between Lf and bacteria results in a bactericidal effect [22]. Lf associates with the lipoprotein of bacterial cells and forms receptor complexes.…”

Are lactoferrin receptors in Gram-negative bacteria viable vaccine targets?

“…To date protein crystallography has yielded structures of the N-terminal domain of LbpB from Moraxella bovis and N. meningitidis (Arutyunova et al 2012;Brooks et al 2014) but no structures of the C-lobe or intact protein have been obtained. Crosslinking coupled to mass spectrometry analyses (XL-MS) supports the orthogonal orientation of the N-lobe and C-lobe of the N. meningitidis LbpB (Ostan et al 2017), and it is likely this orientation is preserved in other LbpBs. LbpB&A can be functionally identified using binding assays, growth assays, and assays measuring the protective effects against cationic peptides associated with the LbpB protein (Yu and Schryvers 2002) (Bonnah et al 1995(Bonnah et al , 1999Morgenthau et al 2012)., with the only distinctive feature of LbpBs being a negatively charged region that is involved in protection against cationic peptides.…”

Biological Functions of the Secretome of Neisseria meningitidis

“…In silico docking experiments indicated that the cap region constitutes the binding site to which lactoferrin binds with its N-lobe (Brooks et al, 2014 ). However, recent biochemical analysis revealed that it preferentially binds the C-lobe of iron-loaded lactoferrin (Ostan et al, 2017 ), just like TbpB binds the C-lobe of iron-loaded transferrin (see above), suggesting a similar role for LbpB as for TbpB in selecting iron-loaded ligands. LbpB appears to have an additional role in protecting the bacteria against the bactericidal activity of lactoferricin (Morgenthau et al, 2012 ).…”

“…Thus, binding of the N-lobe of lactoferrin to the C-lobe of LbpB may prevent proteolysis events that generate lactoferricin. In addition or alternatively, the C-lobe may also be able to bind the free peptide, thus neutralizing its toxic effects (Ostan et al, 2017 ).…”