The Role of the Moraxella catarrhalis CopB Protein in Facilitating Iron Acquisition From Human Transferrin and Lactoferrin.

Clement Chan,Dixon Ng,Anthony B Schryvers

doi:10.3389/fmicb.2021.714815

Abstract

Moraxella catarrhalis is a Gram-negative bacterium that is responsible for a substantial proportion of upper respiratory infections in children and lower respiratory infections in the elderly. Moraxella catarrhalis resides exclusively on the mucosal surfaces of the upper respiratory tract of humans and is capable of directly acquiring iron for growth from the host glycoproteins human transferrin (hTf) and human lactoferrin (hLf). The iron-bound form of these glycoproteins is initially captured by the surface lipoproteins Tf or Lf binding protein B (TbpB or LbpB) and delivered to the integral outer membrane TonB-dependent transport (TBDT) proteins, Tf binding protein A (TbpA) or Lf binding protein A (LbpA). The extraction of iron involves conformational changes in Lf and Tf to facilitate iron removal followed by its transport across the outer membrane by a well characterized process for TBDTs. Surprisingly the disruption of the gene encoding another TBDT, CopB, results in a reduction in the ability to grow on human Tf or Lf. The possibility that this could have been due to an artifact of mutant construction that resulted in the inhibition of TonB-mediated process was eliminated by a complete deletion of the CopB gene. A systematic evaluation of the impact on growth under various conditions by deletions of the genes encoding TbpA, LbpA, and CopB as well as mutations of the iron liganding residues and TonB box region of CopB was implemented. The results indicate that although CopB is capable of effectively acquiring iron from the growth medium, it does not directly acquire iron from Tf or Lf. We propose that the indirect effect on iron transport from Tf and Lf by CopB could possibly be explained by the association of TBDTs at gaps in the peptidoglycan layer that may enhance the efficiency of the process. This concept is supported by previous studies demonstrating an indirect effect on growth of Tf and Lf by deletion of the peptidoglycan binding outer membrane lipoprotein RmpM in Neisseria that also reduced the formation of larger complexes of TBDTs.

Highlights

Moraxella catarrhalis is a Gram-negative bacterium that is an important cause of upper respiratory tract infections in young children and lower respiratory tract infections in elderly patients with chronic obstructive pulmonary disease (De Vries et al, 2009; Murphy and Parameswaran, 2009)
We demonstrate that complete removal of the CopB gene does reduce growth with exogenous human transferrin (hTf) or human lactoferrin (hLf), excluding the mechanism illustrated in Figure 1, and evaluate aspects of the TonB-mediated process that may contribute to this phenomenon
One of the challenges in characterizing the iron acquisition pathways in bacteria like M. catarrhalis that primarily reside on the mucosal surfaces of the upper respiratory tract is that we have a very incomplete understanding of the physiological iron sources

Summary

Introduction

Moraxella catarrhalis is a Gram-negative bacterium that is an important cause of upper respiratory tract infections in young children and lower respiratory tract infections in elderly patients with chronic obstructive pulmonary disease (De Vries et al, 2009; Murphy and Parameswaran, 2009). The surface lipoproteins, Tf and Lf binding protein B (TbpB and LbpB) have an N-terminal anchoring peptide region that allows TbpB or LbpB to extend away from the surface of the outer membrane to capture the iron-loaded form of Tf or Lf (Moraes et al, 2009; Yang et al, 2011; Figure 1, left Panel) This is thought to be important under conditions where the availability of the ironloaded host proteins would be limited, such as on the mucosal surface. The process by which TbpB brings iron-loaded Tf to TbpA is not fully understood, it likely involves conformational changes in the anchoring peptide region and an interaction between a portion of the anchor peptide and TbpA (Yang et al, 2011) so that a ternary TbpB-Tf-TbpA complex is formed for iron removal

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Results

Conclusion