Surface protein antigen implicated in dental caries. A short internal peptide (residues 452-470) has been used as a miniprotein to examine protein folding and stability. Detected as a 185 kDa cell surface protein, but also as 2 proteins in S.mutans culture supernatants of about 150 kDa (antigen I) and 50 kDa (antigen II); antigen II is only seen after proteolysis. Antigen I and II have the same N-terminus but different C-termini. Belongs to the antigen I/II family.