Autoantigenic epitopes of the b polypeptide of SM small nuclear RNP particles

Objective. To analyze the autoantigenic epitopes of the Sm B polypeptide of the U1 small nuclear RNP (snRNP) using complementary DNA (cDNA) clones. Methods. Expression of Sm B fusion proteins in λ phage vectors, immunoblots, immunoprecipitations, and affinity purification of antibodies. Results. Immunoblots using antibodies affinitypurified from B fusion proteins demonstrated that there were cross‐reactive epitopes between the B'/B and A polypeptides of the U1 snRNP. Immunoprecipitation assays suggested that there were at least 3 different autoantigenic epitopes on the B polypeptide that could be classified into 2 general groups based upon autoantibody reactivity. The first group of autoantibodies, which bound 2 separate autoantigenic epitopes (B^U1–1, B^U1–2), participated in immunoprecipitation of the U1 snRNP alone. The second group, which bound the third type of autoantigenic epitope (B^Sm‐1), immunoprecipitated all the abundant Sm snRNPs. Conclusion. There is at least 1 region on the B proteins that is accessible to antibodies only within the structure of the U1 snRNP, as well as a region that is accessible on all Sm snRNPs. These data support the notion that the native U1 RNP, perhaps containing B proteins in a different conformation than those found on other Sm snRNPs, may drive the humoral immune response in systemic lupus erythematosus.