Small nuclear RNA-associated proteins are immunologically related as revealed by mapping of autoimmune reactive B-cell epitopes.

Autoantibodies from a patient with systemic lupus erythematosus, which recognize U1 and U2 small nuclear ribonucleoprotein particles (snRNPs), were used to map B-cell autoepitopes on the U1 snRNP-specific A protein. This protein contains two regions that are highly similar to regions in the U2 snRNP-specific B" protein. A site termed epitope 2 maps in one such region and was found to react with antibodies cross-reactive between A and B". A second site, epitope 1, is situated in a proline-rich region that shows no homology with B". This epitope can bind three different autoantibodies with distinct specificities. Epitope 1-affinity-purified antibodies from different patients react with either (i) the A protein exclusively; (ii) proteins A, B'/B, a synthetic peptide for part of the N polypeptide, and an unidentified protein with a molecular mass of 50 kDa; or (iii) proteins A, B'/B, C, and the N-derived peptide. Comparison of the primary structures of proteins B'/B, N, and C reveals multiple epitope 1-like sequences in all of them. The possibility that these repeating regions act as immunogens in patients with autoimmune disease is discussed.