Autoantibodies to SS-A or Ro are common in patients with the systemic rheumatic diseases Sjogren's syndrome and lupus. In 1984, Wolin and Steitz showed that autoantibodies to SS-A/Ro recognized a family of low abundant ribonucleoprotein particles composed of hY-RNAs and a 60kDa protein component. The latter was the only major protein antigen observed by radiolabeled immunoprecipitation. Recent studies from this laboratory (Ben-Chetrit et. al., 1988) showed that most sera with anti-SSA/Ro as defined by the Ouchterlony method reacted with two different proteins of 60 and 52kDa in immunoblotting suggesting that these two proteins might exist in a complex. Autoantibodies to the 52kDa component were in fact very common in SS-A/Ro autoimmune sera tested (> 80~) and the reason why it had not been noticed was probably a result of the often coexisting autoantibodies to SS-B/La which generated strong signals in immunoblots and was not completely separated from the 52kDa component in the usual SDS PAGE separation system. The two SS-A/Ro components were detected in all human cell lines tested including HeLa, MOLT-4, Raji, and Wil-2 (Ben-Chetrit et. al., 1988).

Using specific SS-A/Ro autoimmune sera, clone CI was selected from a human HepG2 cell IZap cDNA library. Affinity purified antibody to the encoded fusion protein recognized only the 52kDa protein in immunoblotting. Results from northern blot and DNA sequence analysis indicated that C1 encoded 43kDa (= 80~) of the 52kDa SS-A/Ro protein. Southern blot analysis of human peripheral blood DNA digested with EcoRI or HindIII enzymes suggested only i or 2 genes for the 52kDa protein. Using oligonucleotide probes corresponding to the 5'region of the C1 insert, clone 52FL of 1.9 kB was obtained from a MOLT-4 cell cDNA library and it encoded the complete 52kDa protein consisting of a total of 475 amino acid residues with a deduced molecular mass of 54,082.