LUPUS AUTOANTIBODIES TO DOUBLE-STRANDED DNA CROSS-REACT WITH RIBOSOMAL-PROTEIN S1

One cDNA clone (G7) was isolated from a lambda gt11 human liver cDNA l ibrary by the reaction with a serum containing anti-dsDNA Abs and was ligated into pCEX-1 lambda T vector. All the 10 SLE sera with anti-dsD NA, 2 samples of human monoclonal anti-dsDNA (33.C9 and 33.H11), and 2 affinity-purified anti-dsDNA Abs recognized the glutathione S-transfe rase fusion protein expressed by G7 (G7-FP). Ab binding to the recombi nant protein expressed by G7 (G7-RP) and to G7-FP was inhibited comple tely by calf thymus dsDNA. The cDNA was 1314 nucleotides in length and contained an open reading frame encoding 352 amino acids. However, it seems to be a partial length cDNA because the affinity-purified Ab fr om G7-FP recognized only a 104-kDa protein on Western blot using MOLT4 cell extract. The nucleotide sequence of G7 was homologous (99% ident ity) to a cDNA encoding human ribosomal protein (r-protein) S1 homolog ue mRNA, The encoded protein contains repeating residues as a feature of r-proteins S1. Cytoplasmic and nucleolar staining of 33.H11 on indi rect immunofluorescence (IF) using Hep 2 cells was inhibited by both G 7-RP and calf thymus dsDNA. On ELISA, 33.H11 had a higher affinity for G7-RP than for DNA while 33.C9 had a higher affinity for DNA than for G7-RP and binds nuclei on IF. We conclude that G7 encodes a portion o f human r-protein S1 and anti-dsDNA Abs cross-react with this protein.