alpha-Hemolytic oral streptococci are known to possess a family of cell surface cross-reactive proteins termed Ag I/II, having a molecular mass of approximately 180 to 210 kDa. These proteins are implicated in bacterial adherence to various oral tissues, and we showed recently that the SR protein, an I/II Ag-related protein, from Streptococcus mutans OMZ 175 serogroup f possesses Ag mimicry with human IgG. In this study, regions of the SR protein encoding the cross-reactive epitope(s) were analyzed by expressing selected restriction fragments from the cloned sr gene. The three SR-derived polypeptides reacted in ELISA with anti-SR rabbit IgG, whereas only the two polypeptides located along the carboxyl-terminal two thirds of the SR protein reacted with anti-human IgG rabbit IgG. In order to locate more precisely the human IgG-cross-reactive region, we synthesized six peptides, on the basis of the recently determined complete nucleotide sequence of the sr gene. Among these peptides, peptide 2, corresponding to the alanine-rich repeating amino-terminal region, peptide 3, located in the three tandem proline-rich regions, and peptide 6, located near the cell wall-spanning region, were the most interesting in term of antigenicity and immunogenicity. Anti-peptide 2, 3, and 6 rabbit IgG reacted with free SR and with cell wall-associated SR. Peptide 1, located near the amino terminus, was poorly immunogenic. Peptides 4 and 5, located in the putative human IgG-cross-reactive region, were immunogenic; however, anti-peptide 4 rabbit IgG reacted only weakly with SR or human IgG, whereas anti-peptide 5 rabbit IgG reacted strongly with SR and human IgG, and peptide 5 was recognized by anti-SR and anti-human IgG rabbit IgG. These results confirm the cell surface accessibility of this epitope and its potential participation in eliciting, in rabbits, anti-SR IgG cross-reactive with human IgG.
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