X-RAY CRYSTAL-STRUCTURE OF C3D - A C3 FRAGMENT AND LIGAND FOR COMPLEMENT RECEPTOR-2

Activation and covalent attachment of complement component C3 to patho gens is the key step in complement-mediated host defense. Additionally , the antigen-bound C3d fragment interacts with complement receptor 2 (CR2; also known as CD21) on B cells and thereby contributes to the in itiation of an acquired humoral response. The x-ray crystal structure of human C3d solved at 2.0 angstroms resolution reveals an a-a barrel with the residues responsible for thioester formation and covalent att ach ment at one end and an acidic pocket at the other. The structure s upports a model whereby the transition of native C3 to its functionall y active stale involves the disruption of a complementary domain inter face and provides insight into the basis for the interaction between C 3d and CR2.