The polypeptide structure of the Sm and RNP nuclear antigens

Sm and nuclear ribonucleoprotein are ubiquitous nuclear antigens towards which important autoantibodies are directed in systemic lupus erythematosus and other human autoimmune syndromes. Using physicochemical techniques and affinity adsorptions, we have purified the polypeptide components of these antigens. The Sm antigen contained polypeptide chains of 15,000 and 17,000 mol. wt. The RNP antigen, which is known by immunochemical techniques to contain the Sm antigen, had the same two polypeptides as well as a larger one of 85,000 mol. wt. This larger peptide was quite labile and apparently broke down into smaller components with manipulation. In addition, the process of affinity purification of the Sm polypeptides gave a product which had increased positive charge. Amino acid analysis of the Sm polypeptides confirmed the presence of relatively large numbers of basic residues. The purified Sm antigen provided an effective reagent for the investigation of autoreactivity to Sm. The differences in structure from our results and those published by others are probably accounted for by the lability of the constituent polypeptides.