Structural And Functional Studies Of Neisserial Lactoferrin Binding Proteins
Two species of Neisseria, N. meningitidis and N. gonorrhoeae, are obligate human pathogens that cause meningitis and gonorrhea, respectively. Although generally asymptomatic, N. meningitidis can cause invasive meningococcal disease with high mortality rate. Due to emerging antibiotic resistance strains of N. gonorrhoeae, the Centers for Disease Control and Prevention (CDC) have designated it as an urgent threat to public health. Therefore, immediate interventions are required for fight against these Neisserial pathogens. Iron is an essential nutrient for all bacteria, including Neisseria. However, free iron is scarce in human, therefore, Neisseria have evolved to acquire iron from host proteins. These iron acquisition systems are immunogenic and important for infection and are promising therapeutic targets.
In the host, lactoferrin sequesters free iron and limits iron availability to pathogens. However, Neisseria have evolved machinery to hijack iron directly from lactoferrin itself. Lactoferrin binding proteins, LbpA and LbpB, are outer membrane proteins that together orchestrate the acquisition of iron from lactoferrin. Additionally, LbpB serves an additional role in providing protection against host cationic antimicrobial peptides and innate immune response. Despite studies aimed at deciphering the roles of LbpA and LbpB, the molecular mechanisms underpinning iron acquisition and immune protection remain unknown. Here, we investigated the role of the lactoferrin binding proteins in iron acquisition and protection against cationic antimicrobial peptides. We obtained three-dimensional structures of Neisseria LbpA and LbpB in complex with lactoferrin using cryo-electron microscopy and X-ray crystallography. These structures show that both LbpA and LbpB bind to C-lobe of lactoferrin, albeit at distinct sites. Structural analyses show that while lactoferrin maintains its iron-bound closed conformation in the LbpB-lactoferrin complex, it undergoes a large conformational change from an iron-bound closed to an iron-free open conformation upon binding to LbpA. This observation suggest that LbpA alone can trigger the extraction of iron from lactoferrin. Our studies also provide an explanation for LbpB’s preference towards holo-lactoferrin over apo-lactoferrin and LbpA’s inability to distinguish between holo- and apo-lactoferrin. Furthermore, using mutagenesis and binding studies, we show that anionic loops in the C-lobe of LbpB contribute to binding the cationic antimicrobial peptide lactoferricin. Solution scattering studies of the LbpB-lactoferricin complex showed that LbpB undergoes a small conformational change upon peptide binding.Together, our studies provide structural insights into the role of the lactoferrin binding proteins in iron acquisition and evasion of the host immune defenses. Moreover, this work lays the foundation for structure-based design of therapeutics against Neisseria targeting the lactoferrin binding proteins.