CHARACTERIZATION OF AUTOANTIBODIES THAT RECOGNIZE U4/U6 SMALL RIBONUCLEOPROTEIN-PARTICLES IN SERUM FROM A PATIENT WITH PRIMARY SJOGRENS-SYNDROME

We identified autoantibodies that recognize the U4/U6 snRNPs in a serum from a 63-year-old Japanese patient (TT) with primary Sjogren's syndrome. This patient's serum immunoprecipitated U4 and U6 snRNAs exclusively from P-32-labeled HeLa cell extracts and a newly identified 120-kDa protein along with the Sm core proteins (B'/B, D, E, F, and G) from [S-35] methionine-labeled HeLa cell extracts. Immunoblotting demonstrated that only the 120-kDa protein was recognized by this unique serum. In glycerol density gradient centrifugation, the 120-kDa protein reactive with TT serum cosedimented with U4 and U6 snRNAs, suggesting that the 120-kDa protein is a unique component of the U4/U6 snRNP particle. In the same study, the U4/U6 snRNP precipitated by TT serum sedimented only in the lower density, whereas anti-Sm antibodies precipitated U4/U6 snRNAs in a broad range of the gradient. This result suggests the presence of at least two molecular forms of the U4/U6 snRNP particles; larger particles, probably the U4/U5/U6 snRNP complex, and free particles. Thus, the U4/U6 snRNP recognized by TT serum includes the U4 and U6 SnRNAs, with Sm core proteins, and the novel 120-kDa protein, and appears to be a free particle not associated with larger complexes.