Molecular characteristics of SS-B/La and SS-A/Ro cellular antigens

Anti-SS-B/La and anti-SS-A/Ro antibodies coexist in certain patients with connective tissue diseases such as systemic lupus erythematosus or Sjögren's syndrome. The respective antigenic structures with which these autoantibodies bind have not been fully characterized. The present study was conducted to better define these two different cellular antigens. WiL2 cell extracts were used to obtain partially purified SS-B/La and SS-A/Ro antigens. Both were found to be present in most fractions obtained after sequential purification with ammonium sulfate salt precipitation, G-200 gel filtration, DE-52 ion exchange chromatography, and preparative slab gel electrophoresis. However, SS-B/La antigenic activity was also found to be present in some fractions that did not contain detectable SS-A/Ro activity. These findings suggested the existence of two different forms of SS-B/La antigen: one containing the SS-B/La antigen only and the other containing both the SS-B/La and SS-A/Ro antigens. The RNA and protein components of these two ribonuclear protein particles were further defined by immunoprecipitation experiments using 32P-labeled WiL2 cell extract. The SS-B/La antigen was found to be associated with several RNAs while the SS-A/Ro antigen was associated with several other distinct RNAs. Both antibodies precipitated a common 43K molecular weight phosphoprotein. The antigenic peptides of these 2 antibodies were analyzed using an immunoblot system. The SS-B/La antigen was present on a 43K peptide which was unstable and could be degraded to several peptides of lower molecular weight (40K, 38K, 30K), while the SS-A/Ro antigen occurred on a peptide having a molecular weight of about 60K.