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Structure of C3b in complex with CRIg gives insights into regulation of complement activation


The complement system is a key part of the innate immune system, and is required for clearance of pathogens from the bloodstream1. After exposure to pathogens, the third component of the complement system, C3, is cleaved to C3b which, after recruitment of factor B, initiates formation of the alternative pathway convertases2,3,4. CRIg, a complement receptor expressed on macrophages, binds to C3b and iC3b mediating phagocytosis of the particles5, but it is unknown how CRIg selectively recognizes proteolytic C3-fragments and whether binding of CRIg to C3b inhibits convertase activation. Here we present the crystal structure of C3b in complex with CRIg and, using CRIg mutants, provide evidence that CRIg acts as an inhibitor of the alternative pathway of complement. The structure shows that activation of C3 induces major structural rearrangements, including a dramatic movement (>80 Å) of the thioester-bond-containing domain through which C3b attaches to pathogen surfaces. We show that CRIg is not only a phagocytic receptor, but also a potent inhibitor of the alternative pathway convertases. The structure provides insights into the complex macromolecular structural rearrangements that occur during complement activation and inhibition. Moreover, our structure–function studies relating the structural basis of complement activation and the means by which CRIg inhibits the convertases provide important clues to the development of therapeutics that target complement.

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Figure 1: Domain architecture and structure of C3 and the C3b/C3c:CRIg complexes.
Figure 2: CRIg binds to the β-chain of C3b and C3c and inhibits the AP C3 and C5 convertases.
Figure 3: Structure–activity relationship of CRIg mutants.
Figure 4: CRIg inhibits C5 binding to the AP C5 convertase.


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We thank B. de Vos, A. Chan, F. Bazan and H. Spits for critically reading the manuscript, and J. Chinn and B. Appleton for important contributions. We acknowledge the support staff at beamline 5.0.2 at the Advanced Light Source. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences Division of the US Department of Energy at Lawrence Berkeley National Laboratory. Author Contributions K.J.K., JP.Y., K.Y.H. and M.S. contributed equally to this work. K.J.K., L.E. and L.DF. performed the C3 convertase and haemolysis assay, S.A.MC. and W.J.F. contributed to mutant design, K.Y.H. generated CRIg mutants and performed the microwell assays, M.S. and P.E.H. purified the complement proteins, JP.Y. generated CRIg for crystallography, purified the complexes and grew the crystals, C.W. solved the structures and wrote the paper together with M.v.L.C.

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Correspondence to Menno van Lookeren Campagne.

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Coordinates and structure factors have been deposited at the RCSB Protein Databank (2ICC, 2ICE, and 2ICF). Reprints and permission information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Notes

Structure of complement receptor CRIg bound to C3b provides insight into the regulation of complement activation. This file contains Supplementary Methods, Supplementary Table and Supplementary Figure Legends. (DOC 94 kb)

Supplementary Figure 1

Differences in domain arrangement between C3 and C3b. (PDF 152 kb)

Supplementary Figure 2

CRIg has similar affinities for C3b and C3c. (PDF 25 kb)

Supplementary Figure 3a

Sequence alignment of CRIg from different species. (PDF 164 kb)

Supplementary Figure 3b

Sequence alignment of human and mouse C3 (PDF 562 kb)

Supplementary Figure 4

Local differences between C3 and C3b in the vicinity of the CRIg binding site. (PDF 266 kb)

Supplementary Figure 5

CRIg inhibits cleavage of C3 in a fluid-phase C3 convertase assay. (PDF 99 kb)

Supplementary Figure 6

CRIg does not accelerate decay of the C3 convertase. (PDF 93 kb)

Supplementary Figure 7

CRIg does not inhibit the CP convertase. (PDF 23 kb)

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Wiesmann, C., Katschke, K., Yin, J. et al. Structure of C3b in complex with CRIg gives insights into regulation of complement activation. Nature 444, 217–220 (2006).

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