In this study we localized more precisely the salivary glycoprotein-interacting and the human immunoglobulin G (hIgG)-cross-reacting domains on the SR molecule, an antigen I/Il-related protein from S. mutans serotype f. Mapping of the SR molecule with polypeptides expressed by subclones covering the entire molecule and with synthetic peptides demonstrates that the salivary glycoprotein-binding domain is located in the N-terminal alanine-rich repeats of the SR molecule. In order to investigate the degree of conservation of both regions in various oral streptococci, we tested the reactivity of 8 representative strains of the mutans group and 11 nonmutans oral Streptococcus strains (S. anginosus, S. milleri, S. consteUlatus, S. intermedius, S. mitis, S. sanguis, S. gordonii, S. salivarius, and S. mitis strains) with antipeptide antibodies in a whole-cell enzyme linked immunosorbent assay together with colony hybridization analysis using DNA probes designed to map these two regions. All the mutans group strains except S. rattus and the 11 nonmutans streptococcal strains showed a high conservation of the C-terminal part of the SR molecule, especially the hIgG-cross-reacting domain, and less homology for the N-terminal salivary glycoprotein-binding region. Almost all of the sera from patients with rheumatic disease reacted strongly with SR from S. mutans serotype f, PI from S. mutans serotype c, and four peptides located in the hIgG-cross-reacting region and not with peptides located at the C and N termini and in the proline-rich repeats. These results confirm that epitopes located within this region are immunogenic in humans and could lead to the synthesis of natural anti-IgG antibodies.

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Streptococcus mutans surface proteins may be important in immunization against dental caries. We report the existence of an open reading frame of 1,005 bp that lies 1,162 bases upstream of the S. mutans OMZ175 sr gene and that encodes a cell wail-associated protein. This open reading frame codes for 335 amino acid residues. The first 18-amino acid region is predominantly hydrophobic and resembles a signal peptide, and the hydrophobic C-terminal region may function as an anchor to the bacterial cell wall. On the basis of the predicted antigenic determinants of the deduced amino acid sequence, a 16-residue synthetic peptide corresponding to the middle hydrophilic coiled region was synthesized. Antibodies raised against this synthetic peptide reacted with a protein with an apparent Mr of 40,000 that was identified by Western immunoblotting in a cell wall extract from S. mutans OMZ175. The high reactivity in an enzyme-linked immunosorbent assay of the antibodies with whole S. mutans OMZ175 cells showed that this protein was located on the bacterial cell surface. Furthermore, the antipeptide immunoglobulin G recognized an identical determinant on the cell surface of other members of the S. mutans group. However, the function of this protein is not yet known.