A Closer Look at the Adherence of Streptococcus mutans’ GbpC

Objectives: Glucan binding protein C (GbpC) is a known virulence factor of Streptococcus mutans, which is hypothesized to be important for early biofilm formation. The aim of this research is to characterize the biophysical interactions between GbpC and known components present in the biofilm: Salivary agglutinin (SAG/Gp340) and its scavenger rich cysteine rich domains (SRCR₁ and SRCR₁₂₃), Antigen I/II (AgI/II, another surface anchored S. mutans protein), and dextran.
Methods: A BIAcore (surface plasmon resonance) CM5 chip immobilized with SAG, SRCR₁ or SRCR₁₂₃ was used to investigate GbpC’s ability to adhere. Following that, the GbpC analyte mixed with increasing ratios of 5000 Dalton dextran was run across immobilized SRCR₁ to observe any interference to the adherence. In a separate experiment, competitive adherence between GbpC and AgI/II was observed by injections across immobilized SAG. Isothermal titration calorimetry studies were done to shed light on the direct interaction between GbpC and dextran of 9400, 38800, and 70800 Daltons.
Results: In analysis of GbpC binding to immobilized salivary agglutinin constructs, GbpC was found to bind to SAG and SRCRs with nanomolar affinity, demonstrating a function of GbpC beyond its previously reported affinity for glucans. Competitive adherence assays of AgI/II and GbpC across immobilized SAG showed that AgI/II inhibits binding of GbpC by 60.3 +/- 6.7% whereas GbpC has a lesser inhibitory effect on the binding of AgI/II (25.1+/-10%). Isothermal titration calorimetry studies determined that GbpC adheres to dextran with micromolar affinity, where non-cooperative adherence was observed in the interaction of GbpC to dextran sizes 9400, 38800, and 70800 Daltons.
Conclusions: Our findings shed light on a more dynamic role for interactions of Glucan binding protein C as a virulence factor of S. mutans, while also quantifying the adherence of the long-known interaction of GbpC with glucans/dextran.