DNA binds and activates complement via residues 14-26 of the human C1q A chain.

The mechanism by which DNA activates the classical complement pathway was investigated, with emphasis upon the C1q binding sites involved. DNA bound to both the collagen-like and globular regions of C1q. Binding reactivity with DNA was retained after reduction/alkylation and sodium dodecyl sulfate treatment of C1q. DNA bound preferentially to the A chain of C1q. Binding sites for DNA were localized by using synthetic C1q A chain peptides to two cationic regions within residues 14-26 and 76-92, respectively. Peptides 14-26 and 76-92 avidly bound DNA in enzyme-linked immunosorbent and gel shift assays. Peptide 14-26 also precipitated with DNA and blocked its ability to bind C1q and activate C. Replacement of the two prolines with alanines or scrambling the order of the amino acids resulted in loss of ability of peptide 14-26 to inhibit C1q binding and complement activation by DNA; similar investigations showed a sequence specificity for peptide 76-92 as well. These experiments identify C1q A chain residues 14-26 as the major site, and residues 76-92 as a secondary site, through which DNA binds C1q and activates the classical complement pathway, and demonstrate that a peptide identical to residues 14-26 can modulate C1q binding and complement activation by DNA.