Vimentin-cross-reactive epitope of type 12 streptococcal M protein.

The NH2-terminal amino acid sequence of type 12 M protein was determined by automated Edman degradation of a 38-kilodalton polypeptide fragment purified from a limited pepsin digest of intact type 12 streptococci. The sequence of the first 13 amino acid residues of the polypeptide confirmed that predicted by the nucleotide sequence of the mature type 12 M protein. A chemically synthesized peptide copying the NH2-terminal 25 residues, SM12(1-25)C, evoked opsonic antibodies against type 12 streptococci as well as renal glomerular cross-reactive antibodies. The serum from one of six rabbits reacted in immunofluorescence tests with human glomeruli in a mesangial staining pattern. The cross-reactive antibodies were completely inhibited by the immunizing peptide and absorption with type 12 streptococci. Subpeptides of the 25-residue synthetic peptide were without inhibitory effect, suggesting that the cross-reactive antibodies are directed against a conformational epitope of SM12(1-25)C. Anti-SM12(1-25)C antisera reacted specifically with the intermediate filament protein vimentin extracted from mesangial cells. None of the cross-reactions of anti-SM12(1-25)C were inhibited by a synthetic peptide SM1(1-26)C of type 1 M protein, which was previously shown to share a cross-reactive epitope with vimentin. These results indicate that type 12 M protein contains at least one vimentin cross-reactive epitope that is clearly distinct from the tetrapeptide epitope shared with vimentin by type 1 M protein.