Molecular force probe measurement of antigen I/II-matrix protein interactions

Viridans streptococci possess a family of immunologically and structurally related cell-surface proteins, termed antigen I/II, which may function as adhesins and enable oral streptococci to adhere to saliva-coated surfaces and matrix proteins. Here we used atomic force microscopy in the molecular force mode to measure the specific interaction forces between antigen I/II and two matrix proteins, collagen and fibronectin. These matrix proteins provide important binding sites for adherence of oral streptococcal in dentinal caries and endocarditis, respectively. Antigen I/II-coated cantilever tips were brought into contact with collagen- or fibronectin-coated silica coverslips. For the protein I/II-fibronectin interaction experiments, the mean strength of the last ruptures was 216 pN, with most of the detachments located around 125 pN. In antigen I/II-collagen interaction experiments, the mean strength of the last rupture forces corresponded to 136 pN, with the most frequent unbinding force around 75 pN. Thus, our findings definitely suggest that, under the present experimental conditions, antigen I/II binds more strongly to fibronectin than to type I collagen. This might be of relevance for the attachment of viridians streptococci to surfaces exposed to strong hydrodynamic shearing forces under in vivo conditions.