UspA proteins were previously assigned to one of the three main groups: UspA1, UspA2, or UspA2H. Our detailed analysis of 34 UspA amino acid sequences has revealed a remarkable diversity among the three UspA groups as well as the potential to exchange variable motifs between them. Structurally, the interstrain variability appears most evident within the N-terminal region, where the UspA1 and UspA2H proteins possess a wide variability in the number of GGG repeats, each of which forms an antiparallel β-strand, followed by an FAAG motif. This variability will undoubtedly cause large changes in the size of the head domain (Fig. and ), which is otherwise analogous to that of the Yersinia
YadA protein (47
). In stark contrast, the UspA2 N terminus lacks any similarity to the UspA1/UspA2H/YadA structure. This is the greatest difference among these proteins, yet its impact awaits definition of a function attributable to the novel head structure. At the amino acid sequence level, the highly variable U1VR and U2VR regions (Fig. and ) extend the overall diversity by changing the length of the coiled-coil stalk and by conferring different binding phenotypes on the UspA1 and UspA2H proteins.
While the modularity of UspA proteins may facilitate immune escape due to the variation of peptide sequences exposed at the bacterial surface, it also clearly affects the bacterial phenotype. Perhaps the most striking examples of this involve host cellular attachment via CEACAM1 receptors and adherence to Chang cells, with the latter trait appearing to be mediated by binding to the extracellular matrix protein fibronectin. CEACAM binding is common to a variety of UspA1 variants (27
), yet it is clearly not a property observed in all clinical isolates (Fig. ) (11a
). Sequence analysis reveals a direct link between the presence of a complete CEACAM-binding motif and CEACAM binding. While CEACAM binding is not apparent in any of the UspA2 variants tested (11a
), the presence of a portion of the CEACAM-binding domain on UspA2TTA24
clearly illustrates the potential for exchange between UspA classes.
Rather than CEACAM receptor binding, Chang cell binding by M. catarrhalis
is associated with the ability of UspA proteins to bind the extracellular matrix component fibronectin (59
). This activity has been attributed to UspA1 and/or UspA2, yet the majority of UspA1 and UspA2H variants characterized contain the fibronectin-binding motif, whereas only a minority of the UspA2 proteins characterized in this study possessed it. Laminin binding was previously linked to the N-terminal regions of UspA1 and UspA2 (58
); however, ascribing this function to a particular sequence is less clear because the amino acid sequences of laminin-binding UspA protein variants are generally unavailable.
Serum resistance is associated primarily with UspA2 expression (2
), but it is not inherent to all UspA2 variants (2
), and different UspA2 variants appear to have different effects on the complement cascade (5
). For example, while the complement components C4b (6
) and C3 (45
) both bind to UspAs, it is clear that C4b binding is restricted to certain M. catarrhalis
) and that either UspA1 or UspA2 is of primary importance in serum resistances of different strains (6
). The issue of serum resistance in M. catarrhalis
is further complicated by the fact that other mutations have been reported to have an adverse effect on the serum resistance of some strains. The inactivation of the genes encoding CopB (1
), outer membrane protein CD (29
), and outer membrane protein E (10
) was shown to reduce serum resistance. In addition, the expression of at least three genes encoding lipooligosaccharide biosynthesis enzymes (38
) is required for wild-type levels of serum resistance in M. catarrhalis.
To date, however, only the UspA2 protein has been shown to be directly involved in the expression of serum resistance (5
In summary, while various studies have revealed novel UspA protein functions in a prototypical strain, it is clear that the function conferred by different UspA variants may differ widely. Moreover, it is enticing to consider that the various sequence “cassettes” evident in the UspA variants characterized to date may confer heretofore unrecognized functions. Future analyses must obviously consider the sequence of a particular UspA variant being studied and place the results in the context of known allelic differences. Considering the natural genetic competence of M. catarrhalis
), it seems likely that the structural and functional determinants of each UspA protein are phylogenetically fluid, allowing the acquisition of various combinations of binding functions. This combinatorial nature of UspA proteins makes it essential to understand how these different functions interact, with an aim to ascertain whether certain bacterial phenotypes differentiate between asymptomatic commensalism and pathogenesis upon M. catarrhalis