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Structural and functional implications of the alternative complement pathway C3 convertase stabilized by a staphylococcal inhibitor

Nature Immunology volume 10, pages 721–727 (2009)Cite this article

Abstract

Activation of the complement system generates potent chemoattractants and leads to the opsonization of cells for immune clearance. Short-lived protease complexes cleave complement component C3 into anaphylatoxin C3a and opsonin C3b. Here we report the crystal structure of the C3 convertase formed by C3b and the protease fragment Bb, which was stabilized by the bacterial immune-evasion protein SCIN. The data suggest that the proteolytic specificity and activity depend on the formation of dimers of C3 with C3b of the convertase. SCIN blocked the formation of a productive enzyme-substrate complex. Irreversible dissociation of the complex of C3b and Bb is crucial to complement regulation and was determined by slow binding kinetics of the Mg2+-adhesion site in Bb. Understanding the mechanistic basis of the central complement-activation step and microbial immune evasion strategies targeting this step will aid in the development of complement therapeutics.

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Figure 1: SCIN induces the formation of dimeric convertases.
Figure 2: Crystal structure of the C3 convertase C3bBb inhibited by SCIN.
Figure 3: Inhibition of C3bBb by SCIN.
Figure 4: The C3bBb structure derived from the C3bBb-SCIN complex.
Figure 5: The C3b-C3b interface and substrate-binding model.

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Acknowledgements

We thank R. Romijn for help with mammalian protein expression; M. Otten and M. Daha for doing hemolytic assays; P. Lenting for help with Biacore analyses; the European Synchrotron Radiation Facility for synchrotron radiation facilities; and beamline scientists of the European Synchrotron Radiation Facility and the European Molecular Biology Laboratory for assistance. Supported by the Councils for Medical Sciences and Chemical Sciences of the Netherlands Organization for Scientific Research (S.H.M.R., J.A.G.v.S. and P.G.) and the US National Institutes of Health (J.D.L. and P.G.).

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Author notes

  1. Suzan H M Rooijakkers and Jin Wu: These authors contributed equally to this work.

Authors and Affiliations

  1. Medical Microbiology, University Medical Center Utrecht, Utrecht, The Netherlands

    Suzan H M Rooijakkers, Maartje Ruyken, Robert van Domselaar & Jos A G van Strijp

  2. Department of Chemistry, Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Utrecht, The Netherlands

    Jin Wu, Bert J C Janssen & Piet Gros

  3. Van 't Hoff Laboratory for Physical and Colloid Chemistry, Utrecht University, Utrecht, The Netherlands

    Karel L Planken

  4. Department of Pathology & Laboratory Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA

    Apostolia Tzekou, Daniel Ricklin & John D Lambris

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Contributions

J.W. expressed and purified FB and FD; M.R. and S.H.M.R. expressed and purified SCIN and chimeras; M.R. purified C3b; R.v.D., M.R. and S.H.M.R. generated and analyzed complexes and did functional assays; K.L.P. did and analyzed the analytical ultracentrifugation experiments; J.W. crystallized the complex and determined and analyzed the structure; B.J.C.J. helped with structure determination and analysis; A.T. expressed and purified Ba; D.R. did the C3b-FB and C3b-Ba binding studies; S.H.M.R., B.J.C.J., J.W., J.D.L., J.A.G.v.S. and P.G. conceived the experiments; and J.W., S.H.M.R., D.R., J.A.G.v.S. and P.G. wrote the manuscript.

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Correspondence to Piet Gros.

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Rooijakkers, S., Wu, J., Ruyken, M. et al. Structural and functional implications of the alternative complement pathway C3 convertase stabilized by a staphylococcal inhibitor. Nat Immunol 10, 721–727 (2009). https://doi.org/10.1038/ni.1756

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