Analysis of Dextran-binding Domain in Glucan-binding Protein C of Streptococcus mutans

Abstract

Streptococcus mutans, known to be a primary causative agent of dental caries in humans, possesses multiple glucan-binding proteins (Gbps) on its bacterial cell surface, of which 4 types have been identified. Among those, GbpC encoded by the gbpC gene has a strong relationship with cariogenicity in relationship to high dextran-dependent activity, though the mechanism of glucan binding remains unknown. Using bioinformatics analysis and molecular techniques, we examined the dextran-binding domain of GbpC. Based on the 3D structure revealed by a Cn3D macromolecular structure viewer, 5 loops were confirmed as possible dextran-binding domains. Next, truncated recombinant GbpC (rGbpC) encoding each region was produced using a protein expression vector and 5 deletion mutant strains, CDGB1 through CDGB5, were generated. The dextran-binding rate of fragment A, which was truncated rGbpC and included the GB1, GB3, GB4, and GB5 regions, was higher than that of fragment B containing GB2. In addition, the rates of dextran-binding for strains CDGB4 and CD1, gbpC deletion mutants, were significantly lower as compared to the other strains, while those of the other deletion mutants were quite similar to that of the parental strain MT8148. Furthermore, biofilm structures formed by CDGB4 and CD1 were not as pronounced as that formed by MT8148, while those produced by other strains had greater density as compared CD1 biofilm. These results suggest that the dextran-binding domain may be located in GB4, mainly in the 7 amino acid sequences DPTKTIF in the center of gbpC.