Identification and chemical synthesis of a ribosomal protein antigenic determinant in systemic lupus erythematosus.

The characteristics of eukaryotic ribosomal proteins P0, P1, and P2 (P proteins) and their antigenic determinants were studied using the sera of patients with systemic lupus erythematosus (SLE). P0, P1, and P2 were isolated as a macromolecular complex by preparative isoelectric focusing and anion-exchange chromatography in the presence of 6 M urea. The apparent molecular size of the complex was 140 kDa as determined by gel filtration on a Sephadex G-200 column. P0 may, therefore, be the eukaryotic equivalent of Escherichia coli ribosomal protein L10. In addition, all three P proteins were detected in the postribosomal supernatant of HeLa cells, and P0 and P1 were found to be more acidic than their ribosome-bound counterparts. Partial proteolysis experiments revealed that SLE anti-P sera recognized one or both ends of the P2 equivalent protein from Artemia salina (eL12). Sixteen SLE sera containing antibodies to P0, P1, and P2 reacted with a carboxyl-terminal peptide 22 amino acids in length of eL12 and not with an amino-terminal peptide of 20 amino acids. Even though the carboxyl-terminal peptide completely inhibited the ability of the antiserum to react with all three proteins on an immunological blot, the same peptide produced only small decreases in binding of the SLE antibody to the native, nondenatured P proteins. These findings indicate that SLE anti-P antibodies react with a single sequential (linear) antigenic determinant on all three P proteins, but that additional antibodies recognize a conformational determinant(s).