D. Highlight of electron density maps (2Fo-Fc, blue, contour level 1.2σ; Fo-Fc, green and red, contour level 3σ and -3σ, resp.) for the regions of the C3b-regulator complexes showing flexibility. From left to right: the putative site for MCP CCP2, SPICE CCP4, CR1 CCP15, and DAF CCP2. 