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The complex C1 triggers the activation of the Complement classical pathway through the recognition and binding of antigen–antibody complex by its subunit C1q. The globular region of C1q is responsible for C1 binding to the immune complex. C1q can also bind nonimmune molecules such as DNAand sulfated polysaccharides, leading either to the activation or inhibition of Complement. The binding site of these nonimmune ligands is debated in the literature, and it has beenproposed to be located either in the globular region or in the collagen-like regionof C1q, or inboth | Eur. J. Biochem. 270 4714-4720 2003 FEBS 2003 doi 10.1046 j.1432-1033.2003.03870.x Interaction of the C1 complex of Complement with sulfated polysaccharide and DNA probed by single molecule fluorescence microscopy Berangere Tissot1 Regis Daniel1 and Christophe Place2 1Laboratoire Analyse et Environnement Université d Evry France 2Laboratoire de Physique Ecole Normale Superieure de Lyon France The complex C1 triggers the activation of the Complement classical pathway through the recognition and binding of antigen-antibody complex by its subunit C1q. The globular region of C1q is responsible for C1 binding to the immune complex. C1q can also bind nonimmune molecules such as DNA add SLili ritdl polyscccharides leading ei ther to the activation or inhibition of Complement. The binding site of these noaimmune ligands is debated in the literature and it has been proposed to be located either in the globular region or in the collagen-Like region of C1q or in both. Using single molecule fluorescence microscopy and DNA molecular combing as reporters of interactions we have probed the C1q binding properties of T4 DNA and of fucoidan am all ll sulfated fucose-based polysaccharide endowed with potent anticomplementaey activity. We have been able to visualize the binding of C1q as well as of C1 and of the isolated collagen-like region to individual DNA traands mđieadmg that the collagen-like region is the main binding site of DNA. From binding assays with C1r one of the protease components of C1 we concluded that the DNA bmding site on hie collagen-like region is located within the stalk part. Competition experiments between fucoidan and DNAfor the binding of C1q showed that fucoidan binds also to the collagen-like region part of C1q. Unlike DNA the binding of fucoidan to collagen-like region involves interactions with the hinge region that accommodate the catalytic tetramer C1r2-C1s2 of C1. This binding property of fucoidan to C1q provides a mechanistic basis for the .
