The serotype a-EmaA adhesin of Aggregatibacter actinomycetemcomitans does not require O-PS synthesis for collagen binding activity

The Trimeric Autotransporter Adhesin EmaA and Infective Endocarditis

Infective endocarditis (IE), a disease of the endocardial surface of the heart, is usually of bacterial origin and disproportionally affects individuals with underlying structural heart disease. Although IE is typically associated with Gram-positive bacteria, a minority of cases are caused by a group of Gram-negative species referred to as the HACEK group. These species, classically associated with the oral cavity, consist of bacteria from the genera Haemophilus (excluding Haemophilus influenzae), Aggregatibacter, Cardiobacterium, Eikenella, and Kingella. Aggregatibacter actinomycetemcomitans, a bacterium of the Pasteurellaceae family, is classically associated with Aggressive Periodontitis and is also concomitant with the chronic form of the disease. Bacterial colonization of the oral cavity serves as a reservoir for infection at distal body sites via hematological spreading. A. actinomycetemcomitans adheres to and causes disease at multiple physiologic niches using a diverse array of bacterial cell surface structures, which include both fimbrial and nonfimbrial adhesins. The nonfimbrial adhesin EmaA (extracellular matrix binding protein adhesin A), which displays sequence heterogeneity dependent on the serotype of the bacterium, has been identified as a virulence determinant in the initiation of IE. In this chapter, we will discuss the known biochemical, molecular, and structural aspects of this protein, including its interactions with extracellular matrix components and how this multifunctional adhesin may contribute to the pathogenicity of A. actinomycetemcomitans.

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, coexpression 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.