Abstract
Streptococcus suis, one of the most important and prevalent pathogens in swine, presents a major challenge to global public health. HP0197 is an S. suis surface antigen that was previously identified by immunoproteomics and can bind to the host cell surface. Here, we investigated the interaction between HP0197 and the host cell surface glycosaminoglycans (GAGs) using indirect immunofluorescence and cell adhesion inhibition assays. In addition, we determined that a novel 18-kDa domain in the N-terminal region of HP0197 functions as the GAG-binding domain. We then solved the three-dimensional structures of the N-terminal 18-kDa and C-terminal G5 domains using x-ray crystallography. Based on this structural information, the GAG-binding sites in HP0197 were predicted and subsequently verified using site-directed mutagenesis and indirect immunofluorescence. The results indicate that the positively charged residues on the exposed surface of the 18-kDa domain, which are primarily lysines, likely play a critical role in the HP0197-heparin interaction that mediates bacterium-host cell adhesion. Understanding this molecular mechanism may provide a basis for the development of effective drugs and therapeutic strategies for treating streptococcal infections.
Highlights
The function of the Streptococcus suis antigen HP0197 is unknown, but it may be potentially involved in pathogenesis
HP0197 Binds to HEp-2 Cells via Interactions with Cell Surface GAGs—We analyzed the predicted secondary structure of HP0197 and discovered an N-terminal ␣-helix-rich region referred to as the 18-kDa domain based on its molecular mass and a C-terminal G5 domain connected by a proline-rich loop
HP0197 was preincubated with HA, N-acetylglucosamine, glucuronic acid, sialic acid, glucose, galactose, mannose, or heparin, and the binding of HP0197 to the HEp-2 cells was tested by indirect immunofluorescence
Summary
The function of the Streptococcus suis antigen HP0197 is unknown, but it may be potentially involved in pathogenesis. The results indicate that the positively charged residues on the exposed surface of the 18-kDa domain, which are primarily lysines, likely play a critical role in the HP0197-heparin interaction that mediates bacterium-host cell adhesion. Understanding this molecular mechanism may provide a basis for the development of effective drugs and therapeutic strategies for treating streptococcal infections. SrtA (sortase A), an enzyme involved in the covalent linkage of surface proteins to peptidoglycans, cleaves the LPXTG sequence between the Thr and Gly residues and links the protein to the cell wall through the nucleophilic attack of the amino group of the lipid II peptidoglycan precursor [10, 11] This property makes HP0197 a potentially vital virulence factor for S. suis. We initiated a structural and functional study of HP0197, which contributes to the understanding of the pathologic mechanism of S. suis
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