roteins are indeed promising vaccine candidates. MCR_0076, Protective Moraxella catarrhalis Antigens the plug domain of TonB-dependent receptor, is situated within the beta-barrel structure and appears to be more conserved than the barrel. This plug domain is an independent folding subunit blocking the pore until the channel is bound by a ligand and causes the structural and functional differences between these transporters and porins. TonB-dependent receptors have previously been reported to be potential vaccine antigens and important virulence factors and should thus be taken into consideration and analyzed in more detail for M. catarrhalis. The oppA gene encodes an oligopeptide permease that belongs to the ABC transport system. These types of transporters have been shown to play a role in virulence, to be immunogenic and to be potential vaccine candidates. 
The Msp22 antigen MedChemExpress MMAE induced the most significant in vivo protection and was analyzed in vitro in more detail in order to explore its function. Due to its homology to cytochrome c and the presence of a CXXCH motif, known to be involved in heme binding, we tested whether this antigen was indeed a heme binding protein. Our heme staining experiment demonstrated that heme had indeed been covalently attached to the highly soluble Msp22 protein, indicating that Msp22 may exert its function via heme binding. The heme group of type c cytochromes accepts electrons from the bc1 complex and transfers them to the cytochrome oxidase complex. Among other functions, cytochrome c has hemedependent peroxidase activity and plays a role in initiation of apoptosis in more complex organisms. Based on its homology to cytochrome c and its heme binding, Msp22 may also function in the electron transfer via its heme-dependent peroxidase activity. Besides its important role for cytochrome function, heme is also the most abundant source of iron in the human body. Not surprisingly, due to very limited free iron availability in the human host, many pathogens have evolved mechanisms to utilize heme containing 20830712 proteins as iron sources. Recently, two M. catarrhalis proteins have been shown to acquire iron from hemin and heme complexes. Therefore, Msp22 could also be involved in iron acquisition from heme and heme-containing compounds. Interestingly, it was recently suggested that Msp22 has a potential role in divalent ion transport. An investigation into the mechanism of heme binding and the contribution of the CXXCH motif was recently performed for two putative cytochrome c peroxidases of Campylobacter jejuni. While these proteins 17496168 exhibited heme binding, site-directed mutations within the CXXCH motif resulted in unstable proteins excluding ORF MCR_0076 MCR_0196 MCR_0686 MCR_0996 MCR_1003 MCR_1010 MCR_1303 MCR_1416 aa Start-Stop 21160 36485 28558 27148 30375 27386 24679 21152 Length 140 450 531 122 346 360 656 132 No. of non-synonymous/deleted aa 10 32 28 21 7# 21 31 6 No. of isolates 62 63 64 64 64 64 64 64 Sequences were aligned using the Bionumerics algorithm and default settings. Length, length in translated amino acids. #, a single insertion event of 12 amino acids was also observed in a single isolate for this vaccine candidate. doi:10.1371/journal.pone.0064422.t005 12 Protective Moraxella catarrhalis Antigens them from further analysis. Whether this holds true also for M. catarrhalis Msp22 remains to be elucidated. As targets for protective immune responses need to be accessible on the bacterial surface and kn