title: X-ray crystal structure of C3d: a C3 fragment and ligand for complement receptor 2.
abstract: Activation and covalent attachment of complement component C3 to pathogens 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 initiation of an acquired humoral response. The x-ray crystal structure of human C3d solved at 2.0 angstroms resolution reveals an alpha-alpha barrel with the residues responsible for thioester formation and covalent attachment at one end and an acidic pocket at the other. The structure supports a model whereby the transition of native C3 to its functionally active state involves the disruption of a complementary domain interface and provides insight into the basis for the interaction between C3d and CR2.
authors.affiliation: Department of Biochemistry and Department of Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, M5S 1A8, Canada.
authors.forename: B
authors.initials: B
authors.lastname: Nagar
authors.forename: R G
authors.initials: RG
authors.lastname: Jones
authors.forename: R J
authors.initials: RJ
authors.lastname: Diefenbach
authors.forename: D E
authors.initials: DE
authors.lastname: Isenman
authors.forename: J M
authors.initials: JM
authors.lastname: Rini
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D015933:Complement C3d / Q000737:chemistry* / Q000378:metabolism; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D006801:Humans; D008024:Ligands; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017464:Receptors, Complement 3d / Q000378:metabolism*; D016415:Sequence Alignment
chemical_list: D008024:Ligands; D017464:Receptors, Complement 3d; D015933:Complement C3d