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

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Abstract
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 (BU1–1, BU1–2), participated in immunoprecipitation of the U1 snRNP alone. The second group, which bound the third type of autoantigenic epitope (BSm‐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.