Purification and antigenicity of a novel glucan-binding protein of Streptococcus mutans

A novel glucan-binding protein (GBP) having an apparent molecular mass of 59 kDa (GBP59) has been purified from Streptococcus mutans SJ by a combination of affinity chromatography on alpha-1,6-linked glucan, gel filtration chromatography, and ion-exchange chromatography. GBP59 was distinct from the quantitatively predominant S. mutans GBP (GBP74) on the basis of size, elution position in a salt gradient, and antigenicity. Rat antisera to purified GBP59 and GBP74 did not cross-react. GBP59 is apparently immunogenic in humans, since immunoglobulin A (IgA) antibody in 20 of 24 adult parotid saliva samples was shown to react with GBP59 in an enzyme-linked immunosorbent assay. The glucan-binding activity of GBP59 was confirmed by anti-GBP59 immunogold labelling of Sephadex G-50 that had been preincubated with S. mutans culture supernatant. GBP59 could be detected in culture supernatants of all laboratory strains of S. mutans (e.g., Ingbritt), as well as all strains of S. mutans that had been recently isolated from young children. GBP59 was often the only component in protease inhibitor-containing 4-h S. mutans culture supernatants that reacted with human parotid salivary IgA antibody in Western blot (immunoblot) analyses. These studies suggest that GBP59 is a structurally and antigenically distinct S. mutans GBP that can elicit significant levels of salivary IgA antibody in humans.