Thirteen anti-Sm monoclonal antibodies immunoprecipitate the three cytoplasmic snRNP core protein precursors in six distinct subsets

The small nuclear ribonucleoprotein particle (snRNP) common core proteins are the lupus-associated Sm autoantigens. In mouse fibroblasts the seven snRNP core proteins form a particle with a suggested stoichiometry of B₂ [D1,D2(E,F,G)₂] D3. Core particle assembly occurs in the cytoplasm where newly synthesized snRNAs assemble with core proteins stored in three RNA- free complexes of (1) a 6S complex of [D1,D2(E,F,G)₂] (2) a 20S complex of (B,D3 and an unidentified 70 kDa protein) and (3) a 2S-6S complex that minimally contains the B protein. In this report a panel of 13 anti-Sm monoclonal antibodies is shown to immunoprecipitate six different subsets of the cytoplasmic snRNP proteins. Four epitopes are shared by the three aforementioned complexes and five other epitopes are shared by two of the complexes. In addition, the 6S or 20S complexes are apparently disrupted by five of the antibodies. Kinetic studies show that the three cytoplasmic snRNP protein complexes have independent half-lives. These studies provide another approach for characterizing the Sm epitopes. They also complement previous in vitro snRNP assembly studies and suggest that snRNP core assembly occurs by the initial binding of snRNA to the 6S particle followed by addition of the B and D3 proteins.