Dual function of the O-antigen WaaL ligase of Aggregatibacter actinomycetemcomitans

Protein glycosylation is critical to the quaternary structure and collagen binding activity of the extracellular matrix protein adhesin A (EmaA) associated with Aggregatibacter actinomycetemcomitans. The glycosylation of this large, trimeric autotransporter adhesin is postulated to be mediated by WaaL, an enzyme with the canonical function to ligate the O-polysaccharide (O-PS) antigen with a terminal sugar of the lipid A-core oligosaccharide of lipopolysaccharide (LPS). In this study, we have determined that the Escherichia coli waaL ortholog (rflA) does not restore collagen binding of a waaL mutant strain of A. actinomycetemcomitans but does restore O-PS ligase activity following transformation of a plasmid expressing waaL. Therefore, a heterologous E. coli expression system was developed constituted of two independently replicating plasmids expressing either waaL or emaA of A. actinomycetemcomitans to directly demonstrate the necessity of ligase activity for EmaA collagen binding. Proper expression of the protein encoded by each plasmid was characterized, and the individually transformed strains did not promote collagen binding. However, co-expression of the two plasmids resulted in a strain with a significant increase in collagen binding activity and a change in the biochemical properties of the protein. These results provide additional data supporting the novel hypothesis that the WaaL ligase of A. actinomycetemcomitans shares a dual role as a ligase in LPS biosynthesis and is required for collagen binding activity of EmaA. ImportanceThe human oral pathogen A. actinomycetemcomitans is a causative agent of periodontal and several systemic diseases. The organism expresses an adhesin, EmaA, important for the colonization of this pathobiont via collagen binding and biofilm formation. EmaA is suggested to be modified with sugars and the modification is mediated using the same enzymes involved in lipopolysaccharide (LPS) biosynthesis. In this study, evidence is presented which suggests that the WaaL ligase, the enzyme that ligates the O-polysaccharide (O-PS) antigen with a terminal sugar of the lipid A-core oligosaccharide of LPS, is required for the collagen binding activity of EmaA. This finding represents a new paradigm for the posttranslational modification of this type of autotransporter protein.