Letter | Published:

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

Nature volume 444, pages 217220 (09 November 2006) | Download Citation



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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1.

    Complement. First of two parts. N. Engl. J. Med. 344, 1058–1066 (2001)

  2. 2.

    & Relation of putative thioester bond in C3 to activation of the alternative pathway and the binding of C3b to biological targets of complement. J. Exp. Med. 152, 1102–1114 (1980)

  3. 3.

    , & C3 convertase of the alternative complement pathway. Demonstration of an active, stable C3b, Bb (Ni) complex. J. Biol. Chem. 258, 7411–7415 (1983)

  4. 4.

    & Molecular biology and chemistry of the alternative pathway of complement. Adv. Immunol. 29, 1–53 (1980)

  5. 5.

    et al. CRIg: a macrophage complement receptor required for phagocytosis of circulating pathogens. Cell 124, 915–927 (2006)

  6. 6.

    , & Formation of the initial C3 convertase of the alternative complement pathway. Acquisition of C3b-like activities by spontaneous hydrolysis of the putative thioester in native C3. J. Exp. Med. 154, 856–867 (1981)

  7. 7.

    & Structure and biology of complement protein C3, a connecting link between innate and acquired immunity. Immunol. Rev. 180, 35–48 (2001)

  8. 8.

    et al. Structures of complement component C3 provide insights into the function and evolution of immunity. Nature 437, 505–511 (2005)

  9. 9.

    et al. The structure of bovine complement component 3 reveals the basis for thioester function. J. Mol. Biol. 361, 115–127 (2006)

  10. 10.

    et al. Dissection of CR1, factor H, membrane cofactor protein, and factor B binding and functional sites in the third complement component. J. Immunol. 156, 4821–4832 (1996)

  11. 11.

    & Interactions of human complement component C3 with factor B and with complement receptors type 1 (CR1, CD35) and type 3 (CR3, CD11b/CD18) involve an acidic sequence at the N-terminus of C3 alpha'-chain. J. Immunol. 153, 5285–5302 (1994)

  12. 12.

    & Identification of residues within the 727–767 segment of human complement component C3 important for its interaction with factor H and with complement receptor 1 (CR1, CD35). J. Biol. Chem. 274, 5120–5130 (1999)

  13. 13.

    & Structural biology: origins of chemical biodefence. Nature 437, 484–485 (2005)

  14. 14.

    et al. The covalent binding reaction of complement component C3. J. Immunol. 161, 985–990 (1998)

  15. 15.

    , , , & Cloning of Z39Ig, a novel gene with immunoglobulin-like domains located on human chromosome X. Biochim. Biophys. Acta 1492, 522–525 (2000)

  16. 16.

    & Structural insights into the central complement component C3. Mol. Immunol. (in the press).

  17. 17.

    & The C3 convertase of the alternative pathway of human complement. Enzymic properties of the bimolecular proteinase. Biochem. J. 235, 723–730 (1986)

  18. 18.

    , , & A new function of the activated third component of complement: binding to C5, an essential step for C5 activation. Immunology 34, 29–40 (1978)

  19. 19.

    & Functional role of the noncatalytic subunit of complement C5 convertase. J. Immunol. 164, 1379–1385 (2000)

  20. 20.

    et al. Functional analysis of the classical, alternative, and MBL pathways of the complement system: standardization and validation of a simple ELISA. J. Immunol. Methods 296, 187–198 (2005)

  21. 21.

    et al. Mechanism of complement inactivation by glycoprotein C of herpes simplex virus. J. Immunol. 158, 1763–1771 (1997)

  22. 22.

    & C5 convertase of the alternative pathway of complement. Kinetic analysis of the free and surface-bound forms of the enzyme. J. Biol. Chem. 273, 16828–16835 (1998)

  23. 23.

    et al. Decay accelerating activity of complement receptor type 1 (CD35). Two active sites are required for dissociating C5 convertases. J. Biol. Chem. 274, 31160–31168 (1999)

Download references


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.

Author information


  1. Department of Protein Engineering,

    • Christian Wiesmann
    • , JianPing Yin
    • , Wayne J. Fairbrother
    •  & Scott A. McCallum
  2. Department of Immunology,

    • Kenneth J. Katschke
    • , Karim Y. Helmy
    •  & Menno van Lookeren Campagne
  3. Department of Protein Chemistry,

    • Micah Steffek
    •  & Philip E. Hass
  4. Department of Assay Technology, , 1 DNA Way, South San Francisco, California 94080, USA

    • Lizette Embuscado
    •  & Laura DeForge


  1. Search for Christian Wiesmann in:

  2. Search for Kenneth J. Katschke in:

  3. Search for JianPing Yin in:

  4. Search for Karim Y. Helmy in:

  5. Search for Micah Steffek in:

  6. Search for Wayne J. Fairbrother in:

  7. Search for Scott A. McCallum in:

  8. Search for Lizette Embuscado in:

  9. Search for Laura DeForge in:

  10. Search for Philip E. Hass in:

  11. Search for Menno van Lookeren Campagne in:

Competing interests

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

Corresponding author

Correspondence to Menno van Lookeren Campagne.

Supplementary information

Word documents

  1. 1.

    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.

PDF files

  1. 1.

    Supplementary Figure 1

    Differences in domain arrangement between C3 and C3b.

  2. 2.

    Supplementary Figure 2

    CRIg has similar affinities for C3b and C3c.

  3. 3.

    Supplementary Figure 3a

    Sequence alignment of CRIg from different species.

  4. 4.

    Supplementary Figure 3b

    Sequence alignment of human and mouse C3

  5. 5.

    Supplementary Figure 4

    Local differences between C3 and C3b in the vicinity of the CRIg binding site.

  6. 6.

    Supplementary Figure 5

    CRIg inhibits cleavage of C3 in a fluid-phase C3 convertase assay.

  7. 7.

    Supplementary Figure 6

    CRIg does not accelerate decay of the C3 convertase.

  8. 8.

    Supplementary Figure 7

    CRIg does not inhibit the CP convertase.

About this article

Publication history






Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.