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Structure of C3b reveals conformational changes that underlie complement activity


Resistance to infection and clearance of cell debris in mammals depend on the activation of the complement system, which is an important component of innate and adaptive immunity1,2. Central to the complement system is the activated form of C3, called C3b, which attaches covalently to target surfaces3 to amplify complement response, label cells for phagocytosis and stimulate the adaptive immune response. C3b consists of 1,560 amino-acid residues and has 12 domains. It binds various proteins and receptors to effect its functions4. However, it is not known how C3 changes its conformation into C3b and thereby exposes its many binding sites. Here we present the crystal structure at 4-Å resolution of the activated complement protein C3b and describe the conformational rearrangements of the 12 domains that take place upon proteolytic activation. In the activated form the thioester is fully exposed for covalent attachment to target surfaces and is more than 85 Å away from the buried site in native C3 (ref. 5). Marked domain rearrangements in the α-chain present an altered molecular surface, exposing hidden and cryptic sites that are consistent with known putative binding sites of factor B and several complement regulators. The structural data indicate that the large conformational changes in the proteolytic activation and regulation of C3 take place mainly in the first conversion step, from C3 to C3b. These insights are important for the development of strategies to treat immune disorders that involve complement-mediated inflammation.

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Figure 1: Structure of C3b at 4-Å resolution.
Figure 2: Thioester exposure for covalent target attachment.
Figure 3: Exposure and generation of cryptic binding sites.
Figure 4: Proposed model for the conformational pathway of C3.


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We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities; and S.J. Kolodziej and J.K. Stoops (Houston) for providing the native α2-macroglobulin and methylamine-treated α2-macroglobulin EM-reconstruction maps. We thank B. Nilsson (Uppsala), R.B.G. Ravelli (Grenoble) and E.G. Huizinga (Utrecht) for reading the manuscript. This work was supported by a ‘Pioneer’ programme grant to P.G. by the Council for Chemical Sciences of the Netherlands Organization for Scientific Research (NWO-CW) and an NIH grant to J.D.L.

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Correspondence to Piet Gros.

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Coordinates and structure factors of the C3b structure have been deposited in the Protein Data Bank with accession number 2I07. Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Figure

Figure captions for Supplementary Tables 1 and 2, Supplementary Figures 1 to 3 and Supplementary Movie. (DOC 71 kb)

Supplementary Table 1

Diffraction data, refinement statistics and molecular replacement scores for C3b (PDF 28 kb)

Supplementary Table 2

Domain rotation and translation between C3b, C3 and C3c. (PDF 11 kb)

Supplementary Figure 1

Electron density for the individual domains of C3b. (PDF 1581 kb)

Supplementary Figure 2

Proteolytic cleavage sites of factor I mapped on the structure of C3b. (PDF 233 kb)

Supplementary Figure 3

Modeling of native and methylamine α2-macroglobulin based on 25-Å resolution EM reconstruction maps. (PDF 1578 kb)

Supplementary Movie

Conformational transition of C3 into C3b. (AVI 4920 kb)

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Janssen, B., Christodoulidou, A., McCarthy, A. et al. Structure of C3b reveals conformational changes that underlie complement activity. Nature 444, 213–216 (2006).

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