Undesired complement activation is a major cause of tissue injury in various pathological
conditions and contributes to several immune complex diseases. Compstatin, a 13-residue
peptide, is an effective inhibitor of the activation of complement component C3 and thus
blocks a central and crucial step in the complement cascade. The precise binding site on
C3, the structure in the bound form, and the exact mode of action of compstatin are
unknown. Here we present the crystal structure of compstatin in complex with C3c, a major …