cells, would likely not induce compaction of the
C1q arms, and activation may depend on intercomplex proteolysis of surface-bound C1 complexes. Our data suggest that danger pattern
recognition by C1 may lead to proteolysis and
activation within an isolated complex through
a conformational change, as suggested by an observed bending of C1q arms and the arrangement
of proteases. Close interactions observed between
separate C1-IgG complexes, however, suggest that
proteolysis may also result from intercomplex
REFERENCES AND NOTES
1. G. J. Arlaud et al., Immunol. Rev. 180, 136–145 (2001).
2. N. C. Hughes-Jones, B. Gardner, Mol. Immunol. 16, 697–701
3. D. R. Burton et al., Nature 288, 338–344 (1980).
4. L. T. Roumenina et al., Biochemistry 45, 4093–4104
5. C. A. Diebolder et al., Science 343, 1260–1263 (2014).
6. E. E. Idusogie et al., J. Immunol. 166, 2571–2575 (2001).
7. A. R. Duncan, G. Winter, Nature 332, 738–740 (1988).
8. G. L. Moore, H. Chen, S. Karki, G. A. Lazar, MAbs 2, 181–189
9. M. S. Kojouharova et al., J. Immunol. 172, 4351–4358
10. U. Kishore et al., Immunol. Lett. 95, 113–128 (2004).
11. C. Gaboriaud, W. L. Ling, N. M. Thielens, I. Bally, V. Rossi,
Front. Immunol. 5, 565 (2014).
12. T. H. Sharp, F. G. A. Faas, A. J. Koster, P. Gros, J. Struct. Biol.
197, 155–162 (2017).
13. G. Wang et al., Mol. Cell 63, 135–145 (2016).
14. R. N. de Jong et al., PLOS Biol. 14, e1002344 (2016).
15. E. O. Saphire et al., Science 293, 1155–1159 (2001).
16. C. Gaboriaud et al., J. Biol. Chem. 278, 46974–46982
17. A. M. Davies, R. Jefferis, B. J. Sutton, Mol. Immunol. 62, 46–53
18. E. E. Idusogie et al., J. Immunol. 164, 4178–4184
19. M. H. Tao, R. I. Smith, S. L. Morrison, J. Exp. Med. 178,
20. S. M. Canfield, S. L. Morrison, J. Exp. Med. 173, 1483–1491
21. T. E. Michaelsen et al., Eur. J. Immunol. 36, 129–138
22. M. S. Kojouharova, I. G. Tsacheva, M. I. Tchorbadjieva,
K. B. M. Reid, U. Kishore, Biochim. Biophys. Acta 1652, 64–74
23. I. Bally et al., J. Biol. Chem. 284, 19340–19348 (2009).
24. A. E. Phillips et al., J. Immunol. 182, 7708–7717 (2009).
25. U. Venkatraman Girija et al., Proc. Natl. Acad. Sci. U.S.A. 110,
26. M. Budayova-Spano et al., Structure 10, 1509–1519
27. M. Budayova-Spano et al., EMBO J. 21, 231–239 (2002).
28. A. J. Perry et al., J. Biol. Chem. 288, 15821–15829
29. A. R. Gingras et al., Structure 19, 1635–1643 (2011).
30. H. Feinberg et al., EMBO J. 22, 2348–2359 (2003).
31. S. A. Mortensen et al., Proc. Natl. Acad. Sci. U.S.A. 114,
32. S. E. Degn et al., Proc. Natl. Acad. Sci. U.S.A. 111, 13445–13450
Electron density maps are deposited in the Electron Microscopy
Data Bank (EMDB). We gratefully acknowledge helpful
discussions with L. van Bezouwen, D. Meijer, F. Förster (Utrecht,
Netherlands), and S. Scheres (Cambridge, UK), and technical
assistance from C. Schneijdenberg and H. Meeldijk (EM-square,
Utrecht). This research was supported by grants from the
Netherlands Organization for Scientific Research (NWO)
(projects CW 714.013.002, 700.57.010, and STW 13711), the
Institute of Chemical Immunology (NWO 024.002.009), and the
European Research Council (project 233229). This work was
supported by the Netherlands Centre for Electron Nanoscopy
(NeCEN), Leiden (NWO 175.010.2009.001). Conflict of
interests: J.S., F.J.B., B.J.d.K., R.N.d.J., and P. W.H.I.P. are
inventors on patent applications related to complement
activation by therapeutic antibodies and own Genmab stock.
Author contributions: D.U. generated and purified mutant
C1r and C1s, generated C1-IgG16, prepared grids, collected
single-particle micrographs, and processed data. D.U. and P.G.
analyzed single-particle data. R.N.d.J. and D.U. designed Ab
mutants, and B.J.d.K. performed CDC and CD20 binding assays.
S.C.H., R.I.K., A.J.K., and T.H.S. prepared liposomes and grids
and collected, processed, and analyzed tomography data.
D.U., R.N.d.J., T.H.S., P. W.H.I.P., and P.G. wrote the manuscript.
All authors commented on the manuscript. P. W.H.I.P. and
P.G. conceived the project. Density maps of C1-IgG1 complexes
on liposomes and soluble C1-IgG16 complexes are deposited
into the EMDB under accession codes EMD-4231 and
EMD-4232, respectively. The model of gC1q-Fc derived at
7-Å resolution is deposited in the Protein Data Bank with
accession code 6FCZ.
Materials and Methods
Figs. S1 to S7
27 July 2017; accepted 10 January 2018
Fig. 4. Structural model of C1 fitted into C1-IgG16 density. (A) Model for C1q-A, -B, and -C hexamer
indicating collagen-like segments forming a N-terminal stalk region, six collagen-like triple helices, and
C-terminal trimeric gC1q modules. Shown are top and side views (left and middle) of C1q and side, sliced
through top and bottom view (third column, left to right) of the C1q stalk region. Numbering of each
C1q arm is as in Fig. 2. (B) Model for C1r and C1s heterotetramer showing C1r CUB1-EGF-CUB2 (blue) and
C1s CUB1-EGF-CUB2-CCP1 domains (cyan). Shown are (left) top view and (top right) side view at lower
contour level, with the latter revealing density for the CCP1 domain of C1r. An illustration of the domain
arrangement is shown for clarity (bottom right). (C) Overall C1-IgG16 models in density. CCP2-SP domains
lacking density have been added by using orientations derived from crystal structures.