Fig. 3.

C1 binding to hexameric IgM platforms. (A) Model of C1qr₂s₂ in density (truncated below the IgM Fc platform) bound to hexameric IgM showing the orientations of C1r and C1s. (Below) Central slices through the model. The Inset displays the CUB1 and CUB2 domains of the two antiparallel C1r molecules binding to C1q-collagen helices. (B) C1q has six collagen legs spiraling down to hexagonally arranged globular head domains. (C) Binding of heterotrimeric gC1q units to DLPSP residues (orange) of the FG loop of Fc Cµ3 (SI Appendix, Fig. S6).