IMMUNOREACTIVITY AND CONFORMATION OF THE P-P-G-M-R-P-P REPETITIVE EPITOPE OF THE SM AUTOANTIGEN

Anti-Sm antibodies are usually considered highly specific for systemic lupus erythematosus (SLE), while anti-U1 RNP antibodies are found in high titers in patients with mixed connective tissue disease (MCTD). T he sequence P-1-P-G-M-R-P-P-7, present in three copies in the Sm (U1-U 6 RNA-protein complex) autoantigen, is an important functional domain of the antigenic determinants. The immunoreactivity of this proline-ri ch repetitive epitope was investigated by testing sera with various au toantibody specificities for reactivity against this epitope, as well as its conformational properties by means of 1D and 2D (1)HNMR spectro scopy. It was found that the P-P-G-M-R-P-P epitope is recognized mainl y by anti-U1RNP and/or anti-Sm positive sera, but also by anti-Ro(SSA) (hY1RNA-protein complex) and anti-La(SSB) (hY1RNA-protein complex) po sitive sera, although these sera are negative for anti-U1RNP and anti- Sm. Conformational analysis of the proline-rich epitope in DMSO-d(6) s olution obtained from lyophilized aqueous solution at pH 5 showed the presence of at least three conformers. The main conformer A (62%) is s tabilized by an ionic interaction between the guanidinium and the C-te rminal carboxylate groups, and the Pro(6)-Pro(7) peptide bond adopts t he cis form. A type II beta-turn is also present in the N-terminal seq uence (Pro(1)-Pro-Gly-Met(4)-) of this conformer. Conformer B (21%) is also stabilized by a similar ionic interaction, as in conformer A, wh ile the NMR data indicate the absence of a folded structure in the N-t erminal tetrapeptide of this conformer. Conformer C (17%) adopts a com pletely extended structure. The multiple conformers of the P-P-G-M-R-P -P may offer some explanation for the reactivity of sera with various autoantibody specificities against this epitope. (C) Munksgaard 1996.