X-ray Crystal Structure of C3d: A C3 Fragment and Ligand for Complement Receptor 2

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Science  22 May 1998:
Vol. 280, Issue 5367, pp. 1277-1281
DOI: 10.1126/science.280.5367.1277

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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 α-α 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.

  • * These authors contributed equally to this work.

  • Present address: Centre for Virus Research, Westmead Hospital, University of Sydney, Westmead, NSW 2145, Australia.

  • To whom correspondence should be addressed at the Department of Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, Canada, M5S 1A8. E-mail: james.rini{at}

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