A sortase-deficient mutant of Streptococcus mutans was prepared by insertional inactivation of a sortase gene (srtA). The srtA mutant was defective in cell wall-anchoring of two surface proteins 200 and 75 kDa in size. A previous study has shown that the 200 kDa protein is a surface protein antigen PAc and that the sortase catalyzes cell wall-anchoring of PAc in S. mutans. In this study another surface protein 75 kDa in size was examined by immunologic and physiologic methods. Western blot analysis with a specific antiserum showed that the 75 kDa protein was a surface protein, glucan-binding protein C. The protein was overexpressed under a stress condition including a sublethal concentration of tetracycline. The srtA mutant cells also lost the ability of dextran-dependent aggregation. These results suggest that the S. mutans sortase mediates cell wall-anchoring of the glucan-binding protein C and dextran-dependent aggregation of this organism.