Characterization of Moraxella catarrhalis iron uptake proteins and their application in vaccine design

Abstract

The uptake of iron is essential for Gram-negative opportunistic pathogens to colonize and infect the host. To obtain iron from the host iron carriers, transferrin (Tf) and lactoferrin (Lf), Moraxella catarrhalis utilizes the specialized receptors Tf-binding protein (Tbp) A/B and Lf-binding protein (Lbp) A/B, for the extraction of iron from Tf/Lf and subsequent transport through the outer membrane. This process is TonB-dependent and has been well characterized in a number of Gram-negative species. In this study, we addressed the gaps of knowledge in M. catarrhalis iron uptake. We initially focussed on the periplasmic transport pathways that are involved in the iron transport through the periplasm and inner membrane. We solved the crystal structures of M. catarrhalis ferric binding protein A (FbpA) in the iron-holo and iron-apo state and identified the residues that are responsible for iron binding. We generated a computational model of M. catarrhalis AfeA and identified putative iron-binding residues. The single gene deletion of fbpA or afeA showed that M. catarrhalis requires FbpA, but not AfeA, for growth on human Tf or Lf. In our second aim, we focused on determining the mechanism behind a phenomenon where the deletion of copB has been observed to alter the bacteria’s ability to utilize Tf/Lf as iron sources and how it fits into our understanding of TonB-dependent transport. We eliminated the possibility that this phenomenon could be caused by artifacts from the original mutant design and then have shown that CopB does not directly take up iron from Tf and Lf, but rather increases the efficiency of TbpA/LbpA mediated transport. We have also identified the structural features of CopB that might be involved in this process, including the putative iron binding residues and TonB box of CopB. Based on these results, we suggested an updated model of TonB-dependent transport where the clustering of receptors can improve substrate transport efficiency. These two studies of this project have improved our understanding of M. catarrhalis iron acquisition and revealed the vulnerabilities of the bacteria that can be exploited in the future M. catarrhalis vaccine design.