Conservation of salivary glycoprotein-interacting and human immunoglobulin G-cross-reactive domains of antigen I/II in oral streptococci.

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/II-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. constellatus, 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, P1 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.

A 40-kilodalton cell wall protein-coding sequence upstream of the sr gene of Streptococcus mutans OMZ175 (serotype f).

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 wall-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.

Molecular characterization of the gene sr of the saliva interacting protein from Streptococcus mutans OMZ175.

The saliva interacting protein (74KSR) from Streptococcus mutans serotype f, which is immunologically related to antigen I/II from serotype c, also termed B, P1, PAc, is probably involved in the adherence process of Strep. mutans to the tooth surface. A solid-phase adherence assay showed that 38% of the binding of salivary glycoproteins to Strep. mutans OMZ175 was due to this protein. We have cloned and sequenced the 74KSR gene (sr), which produces a recombinant protein (rec195K) with a relative molecular mass of 195,000, as estimated by SDS-PAGE. The strong immunological relationship and functional identity of the 74KSR and rec195K indicate that the Mr 195K protein is probably a precursor form, post-translationally processed, of the 74KSR produced in Strep. mutans. The gene sr consists of 4667 bp and codes for a 171,177 Mr protein. Biochemical features of the protein (density in proline residues and hydrophobicity) may explain the difference observed between the SDS-PAGE estimated molecular mass of the immature protein and the one deduced from the nucleotide sequence. Intra-species hybridization experiments using three contiguous restriction fragments isolated from gene sr as probes showed that the sequence is highly similar in strains from serotypes c and e. We have also shown that a fraction of the heart specific antibodies induced in rabbits during immunization with the 74KSR or rec195K reacts predominantly with human IgG and suggest the hypothesis of antigen mimicry as an explanation for the production of anti-IgG autoantibodies. It will be of great importance to identify the cross-reactive epitopes within the molecule before considering its use in protective immunization against oral streptococci.