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

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.

Hybrid antigens expressing surface loops of BauA from Acinetobacter baumannii are capable of inducing protection against infection

“…Recently, the structure of BauA was solved ( 17 ) and one of the structures shows BauA as a trimer where the loop 5 extends into the center of the complex and loops 8 and 7 are on the outer edges of the complex ( Figure 6 ). Although the trimer could simply be an artefact of the crystallization process, it is important to consider that TonB-dependent processes must occur in gaps in the peptidoglycan layer that would foster some clustering of TBDTs and TonB-ExbB-ExbD complexes ( 35 ). In addition, clustering of the proteins involved in TonB-dependent transport could also enhance the efficiency of the process.…”

Structural and functional insights into iron acquisition from lactoferrin and transferrin in Gram-negative bacterial pathogens

“…FetA in Neisseria , CopB in M. catarrhalis and IrpA in M. bovis are reasonable homologues of each other (44% identity, Table 1 ) and their structural models preserve the overall canonical features. The third extracellular loop contains a REEF domain at the beginning that has been shown to be important for iron acquisition from Tf and Lf in CopB (Chan et al 2021 ). All three proteins have adjacent His and Tyr amino acid residues that are positioned for effective iron binding and were also shown to be important for iron acquisition from Tf and Lf by CopB (Chan et al 2021 ).…”

“…The third extracellular loop contains a REEF domain at the beginning that has been shown to be important for iron acquisition from Tf and Lf in CopB (Chan et al 2021 ). All three proteins have adjacent His and Tyr amino acid residues that are positioned for effective iron binding and were also shown to be important for iron acquisition from Tf and Lf by CopB (Chan et al 2021 ). FetA and CopB differ in that it is loop 5 in FetA and loop 3 in CopB that vary substantially in size whereas structural models predict that it is loop 2 in IrpA that is particularly large (Fig.…”

“…As expected, the process of iron acquisition from Lf by the two component Lf receptor complex is similar to that of the Tf receptor although the presence of the negatively charged regions on LbpB have made the experimental studies more challenging (Morgenthau et al 2013 ; Yadav et al 2019 ). In M. catarrhalis and Moraxella bovis , previous experiments have shown that the deletion of an iron-regulated TBDT, CopB and IrpA respectively, interferes with the ability to grow on Tf and Lf as a sole source of iron (Chan et al 2021 ; Kakuda et al 2003 ; Aebi et al 1996 ). The homologue in the pathogenic Neisseria species, FetA or FrpB, was originally identified as a siderophore receptor (Carson et al 2000 , 1999 ), but there have not been any reports on its impact on iron acquisition from Tf or Lf.…”

“…Structural studies with FrpB (Saleem et al 2013 ) clearly demonstrate that it directly binds to the iron atom which is not typical for many siderophore receptors and raises the question of whether it primarily transports iron. Similar to the pathogenic Neisseria species, the Tf and Lf receptors in M. catarrhalis have been identified and functionally characterized (Luke and Campagnari 1999 ; Bonnah et al 1999 ) and the mechanism by which CopB affects iron acquisition from Tf and Lf has been shown to be indirect (Chan et al 2021 ). To have a more complete understanding of this phenomenon and how it compares to the situation in Neisseria it would be important to compare the roles of the periplasmic binding proteins in these two species.…”