Identification of antigenic epitopes in an alanine-rich repeating region of a surface protein antigen of Streptococcus mutants.

A surface protein antigen (PAc) of Streptococcus mutans with a molecular mass of 190 kDa is considered to play an important role in the initial attachment of this streptococcus to the tooth surface. Two internal repeating amino acid sequences are present in the PAc molecule. One repeating region located in the N-terminal region is rich in alanine (A-region), and the other, located in the central region, is rich in proline (P-region). To identify antigenic epitopes on the A-region of the PAc protein, 82 sequential overlapping synthetic decapeptides covering one of the repetitive units of the A-region were synthesized. In the epitope scanning analyses using murine antisera raised against recombinant PAc (rPAc), multiple antigenic epitopes were found in the repetitive unit of the A-region, and some of them reacted with antisera to rPAc from BALB/c, B10, B10.D2, and B10.BR mice. In particular, a peptide YEAALKQY (residues 366 to 373) was recognized by anti-rPAc sera from all four strains of mice. The reactivities of anti-rPAc sera in the epitope scanning were confirmed by using a purified synthetic peptide, NAKATYEAALKQYEADLAA (corresponding to residues 361 to 379). Furthermore, antisera against a surface protein antigen PAg (SpaA) of Streptococcus sobrinus from BALB/c mice reacted strongly to residues 330 to 337, 362 to 369, and 366 to 373 of the PAc protein by the epitope scanning analysis. An AKATYEAALKQY (residues 362 to 373 of the PAc protein)-like sequence, AKANYEAKLAQY, was found within the A-region of S. sobrinus PAg, suggesting that the amino acid sequences AKA-YEA and YEA-L-QY may be major cross-reactive epitopes of the S. mutans PAc protein and the S. sobrinus PAg protein.