The purification, characterization and amino acid analysis of nuclear ribonuclear protein and Sm antigens reacting with human autoimmune sera.

'Monospecific' antibodies directed against nuclear ribonucleoprotein (nRNP) and Smith (Sm) antigens from the sera of patients with mixed connective tissue disease (MCTD) and systemic lupus erythematosus (SLE) were used to isolate and purify the respective antigens from calf thymus nuclear extracts. The antibodies were allowed to interact with the nuclear antigens and the resulting complexes were purified by Protein-A-Sepharose and poly-U-Sepharose chromatography. When the Protein-A-Sepharose antigen-antibody complex was sequentially eluted with 20% and 60% ethylene glycol, 2 fractions could be identified, one containing antigenically active Sm and the other antigenically active Sm and nRNP. The nRNP fraction contained 5 major proteins with mol. wts ranging from 38,000 to 150,000. The Sm antigen had a mol. wt of 68,000 and was noted to 'co-purify' with the nRNP antigen. Amino acid analysis of the purified proteins demonstrated a high content of glycine, serine and acidic amino acid residues similar to previously described core proteins of heterogeneous nuclear RNP (hnRNP), a precursor to mammalian messenger RNA. Analysis of the RNA moiety following nuclease phosphorylase reactions demonstrated the presence of a RNA polynucleotide with a 'capped' structure at the 5' terminus, a feature consistent with the concept that the nRNP antigen is part of the hnRNP complex.