Molecular characterization of the SM and RNP nuclear antigens

Patients with systemic lupus erythematosus (SLE) often possess antibodies against two nuclear antigens, designated "Sm" and "RNP". The exact relationship between Sm and RNP is not clear; the present study was conducted to define these two different nuclear antigens. Rabbit thymus extracts were used to obtain purified Sm/RNP complex and free Sm antigens by using a combination of 25-60% ammonium sulfate precipitation, DEAE-Sephacel and hydroxylapatite chromatography. By using the separated antigens, sera characterized as anti-Sm, anti-Sm/RNP and anti-RNP could be distinguished by enzyme-linked immunosorbent assay (ELISA). Anti-Sm and/or anti-RNP antibodies were detected in 32 (52%) of 62 sera from patients with SLE. Of these 32, 6 contained anti-Sm only, 10 contained anti-RNP only and 16 had both. When HeLa nuclear extracts were used as antigens by immunoblotting, sera with anti-RNP reacted primarily with 2 polypeptides of 68 and 45 KD; sera with anti-Sm recognized mainly on 2 polypeptides of 26 and 14 KD; sera with anti-Sm/RNP recognized both groups. When purified Sm/RNP complex from rabbit thymus extracts was used as antigen by immunoblotting, sera with anti-RNP reacted with 68 KD protein and putative degradation products of the 68 KD protein. (major: 63 KD, 45 KD, 40 KD; minor: 54-47 KD); sera with anti-Sm recognized 14 KD protein; sera with anti-Sm/RNP reacted with both groups. Although Sm and RNP can exist as a Multimolecular complex, the epitopes recognized by anti-Sm and anti-RNP differ greatly. The Sm determinants reside primarily on proteins of 26 KD and 14 KD, whereas the RNP determinants reside mainly on a protein of 68 KD.