Structural insight into the lactoferrin receptors from pathogenic Neisseria

Abstract

Neisseria are pathogenic bacteria that cause gonorrhea, septicemia, and meningitis. Like other pathogenic bacteria, Neisseria must acquire iron for survival from their local environment within the human host. Instead of secreting siderophores to scavenge iron, Neisseria steal iron from human iron binding proteins such as hemoglobin, transferrin and lactoferrin for survival. Recently we reported the crystal structures of the N. meningitidis transferrin receptors TbpA and TbpB, as well as the structures of apo and holo human transferrin. We also analyzed these proteins using small angle X-ray scattering and electron microscopy to provide the molecular details explaining how Neisseria are able to interact with and extract iron from transferrin. Here, we utilize the structural reports, as well as the recently reported structure of the N-lobe of LbpB from Moraxella bovis, to assemble improved 3D homology models for the neisserial lactoferrin import receptors LbpA and LbpB, both of which are important vaccine targets against N. meningitidis. We then analyzed these models to gain structural insights into the lactoferrin-iron import system and form a mechanistic model fashioned in parallel to the homologous transferrin-iron import system.

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

“…In this respect it is interesting to note that a docking model for the human Lf -N. meningitidis LbpB interaction using the LbpB N-lobe structure (Brooks et al 2014) involves the N-terminal lobe of Lf, including the lactoferricin/lactoferrampin region that was identified in the positive selection analysis. For the alternate model that has been proposed (Noinaj et al 2013), which involves the Lf C-lobe, other bacterial components would have to be invoked in order to explain the positive selection analysis results that primarily impact the Lf N-lobe. These could potentially include the PspA protein from Streptococcus (Shaper et al 2004) and the negatively charged regions of the LbpB C-lobe (Morgenthau et al 2014a), although it is difficult to envision how the latter could confer species specificity.…”

“…The combination of biochemical studies and structural studies of the meningococcal TbpTf complexes (Calmettes et al 2012;Noinaj et al 2012) have led to the general expectation that both TbpB and TbpA bind to the Tf C-lobe for the iron removal process, likely a reflection of the anticipated preferential binding of iron to the C-lobe under physiological conditions. The demonstrated binding of the meningococcal LbpA to the Lf C-lobe (Wong and Schryvers 2003) and binding of meningococcal LbpB N-lobe to Lf ( Figure II) tends to extend the generalization to the LbpB-LbpA receptors.…”

“…The crystal structures of TbpB Moraes et al 2009), TbpB complexed with Tf (Calmettes et al 2012) and TbpA complexed with Tf (Noinaj et al 2012) have provided additional insights into the iron acquisition process. The presence of a 40 amino acid anchor peptide on the N-terminus provides the ability of TbpB to extend substantially from the outer membrane surface for effective capture of iron-loaded Tf (Moraes et al 2009).…”

“…During the formation of the ternary complex, the Tf Clobe switches from a closed to a partially open conformation that facilitates the removal of iron and its subsequent transport across the outer membrane (Noinaj et al 2012). …”

“…However, the expression of NalP is phase variable which provides the opportunity for efficient iron acquisition under appropriate conditions and release of LbpB when this would be advantageous (Oldfield et al 2013). Although there is little or no direct experimental data to base it on, two recent publications have proposed models for the LbpB-Lf interaction, one predicting an interaction analogous to the TbpB-Tf interaction (Noinaj et al 2013) and the other using the recent structures of LbpB N-lobe from M. bovis and D r a f t N. meningitidis to prepare docking models in which the LbpB N-lobe binds to the Lf N-lobe (not C-lobe) (Arutyunova et al 2012;Brooks et al 2014). …”