Interaction of the C1 complex of complement with sulfated polysaccharide and DNA probed by single molecule fluorescence microscopy.

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 and 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 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, an algal sulfated fucose-based polysaccharide endowed with potent anticomplementary 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 strands, indicating 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 binding site on the collagen-like region is located within the stalk part. Competition experiments between fucoidan and DNA for 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 anticomplementary activity of the sulfated polysaccharide.