O-Polysaccharide Glycosylation Is Required for Stability and Function of the Collagen Adhesin EmaA of Aggregatibacter actinomycetemcomitans

Abstract

Aggregatibacter actinomycetemcomitans is hypothesized to colonize through the interaction with collagen and establish a reservoir for further dissemination. The trimeric adhesin EmaA of A. actinomycetemcomitans binds to collagen and is modified with sugars mediated by an O-antigen polysaccharide ligase (WaaL) that is associated with lipopolysaccharide (LPS) biosynthesis (G. Tang and K. Mintz, J. Bacteriol. 192:1395-1404, 2010). This investigation characterized the function and cellular localization of EmaA glycosylation. The interruption of LPS biogenesis by using genetic and pharmacological methods changed the amount and biophysical properties of EmaA molecules in the outer membrane. In rmlC and waaL mutant strains, the membrane-associated EmaA was reduced by 50% compared with the wild-type strain, without changes in mRNA levels. The membrane-associated EmaA protein levels were recovered by complementation with the corresponding O-polysaccharide (O-PS) biosynthetic genes. In contrast, another trimeric autotransporter, epithelial adhesin ApiA, was not affected in the same mutant background. The inhibition of undecaprenyl pyrophosphate recycling by bacitracin resulted in a similar decrease in the membrane-associated EmaA protein. This effect was reversed by removal of the compound. A significant decrease in collagen binding activity was observed in strains expressing the nonglycosylated form of EmaA. Furthermore, the electrophoretic mobility shifts of the EmaA monomers found in the O-PS mutant strains were associated only with the membrane-associated protein and not with the cytoplasmic pre-EmaA protein, suggesting that this modification does not occur in the cytoplasm. The glycan modification of EmaA appears to be required for collagen binding activity and protection of the protein against degradation by proteolytic enzymes.