Fig. 4.

Proposed mechanism of assembly of C1. (A) C1r₂C1s₂ adopts an extended structure in solution (Left), in which the two C1r-C1s dimers are linked by a central interaction between the catalytic domains of C1r. It folds up (Middle) to form a more compact structure to bind to the six collagenous stems of C1q (Right). Contacts between the catalytic domains of C1r prevent one C1r polypeptide from activating its partner. Black dots show the positions of the binding sites for the collagen-like domains of C1q (10). (B) Model of C1 generated by rigid-body fitting to SAXS data. C1q is in gray, C1r is in green, and C1s is in blue. (C) Rigid-body fit to scattering data SASDB38. I(s) is the intensity, and s is the scattering vector. The χ² value for the fit is 2.9. The fit is shown as a solid line, and the residuals to the fit are shown below with a scale of ±0.5.