Tsikaris V, Vlachoyiannopoulos P G, Panou-Pomonis E, Marraud M, Sakarellos C, Moutsopoulos H M, Sakarellos-Daitsiotis M
Department of Chemistry, University of Ioannina, Greece.
Int J Pept Protein Res. 1996 Oct;48(4):319-27. doi: 10.1111/j.1399-3011.1996.tb00848.x.
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). The sequence P1-P-G-M-R-P-P7, present in three copies in the Sm (U1-U6 RNA-protein complex) autoantigen, is an important functional domain of the antigenic determinants. The immunoreactivity of this proline-rich repetitive epitope was investigated by testing sera with various autoantibody specificities for reactivity against this epitope, as well as its conformational properties by means of 1D and 2D 1HNMR spectroscopy. It was found that the P-P-G-M-R-P-P epitope is recognized mainly 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) positive sera, although these sera are negative for anti-U1RNP and anti-Sm. Conformational analysis of the proline-rich epitope in DMSO-d6 solution obtained from lyophilized aqueous solution at pH 5 showed the presence of at least three conformers. The main conformer A (62%) is stabilized by an ionic interaction between the guanidinium and the C-terminal carboxylate groups, and the Pro6-Pro7 peptide bond adopts the cis form. A type II beta-turn is also present in the N-terminal sequence (Pro1-Pro-Gly-Met4-) of this conformer. Conformer B (21%) is also stabilized by a similar ionic interaction, as in conformer A, while the NMR data indicate the absence of a folded structure in the N-terminal tetrapeptide of this conformer. Conformer C (17%) adopts a completely 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.
抗Sm抗体通常被认为对系统性红斑狼疮（SLE）具有高度特异性，而抗U1 RNP抗体在混合性结缔组织病（MCTD）患者中呈高滴度。Sm（U1 - U6核糖核蛋白复合物）自身抗原中以三个拷贝形式存在的序列P1 - P - G - M - R - P - P7是抗原决定簇的一个重要功能域。通过检测具有各种自身抗体特异性的血清对该表位的反应性，研究了这个富含脯氨酸的重复表位的免疫反应性，并借助一维和二维1H NMR光谱研究了其构象性质。发现P - P - G - M - R - P - P表位主要被抗U1RNP和/或抗Sm阳性血清识别，但也被抗Ro(SSA)（hY1核糖核蛋白复合物）和抗La(SSB)（hY1核糖核蛋白复合物）阳性血清识别，尽管这些血清抗U1RNP和抗Sm为阴性。对从pH 5的冻干水溶液在DMSO - d6溶液中获得的富含脯氨酸表位的构象分析表明至少存在三种构象体。主要构象体A（62%）通过胍基和C末端羧基之间的离子相互作用而稳定，并且Pro6 - Pro7肽键呈顺式构象。在该构象体的N末端序列（Pro1 - Pro - Gly - Met4 -）中也存在一个II型β - 转角。构象体B（21%）也通过与构象体A中类似的离子相互作用而稳定，而NMR数据表明该构象体的N末端四肽中不存在折叠结构。构象体C（17%）采用完全伸展的结构。P - P - G - M - R - P - P的多种构象体可能为具有各种自身抗体特异性的血清对该表位的反应性提供一些解释。
