Fig. 5

Inhibitors reveal two important interaction sites for C5 with surface-bound C3b. a Left, schematic representation of the proposed interaction between substrate C3 and the alternative pathway (AP) C3 convertase (based on crystal structure [8]). Right, binding of C5 to CVFBb (based on the CVF:C5 crystal structure [24]). CVF is a potent C3b homologue that lacks the thioester domain and forms stable C5 convertases in solution. b Structural model of the previously proposed AP C5 convertase. The C3/C5 convertase (C3bBb) is shown in ribbon representation, with C3b in gray and Bb in orange, respectively. C5 (green) is shown as a molecular surface, with residue involved in eculizumab (magenta) and SSL7 (cyan) binding colored on the surface. The left and right representations represent the same complex rotated 180° about the vertical axis. c C5 conversion on C3b-coated beads in the absence or presence of 20 μg/ml C5 inhibitors (SSL7, SSL7ΔC5, eculizumab) as determined by calcium mobilization of U937-C5aR cells. d C5 conversion by soluble CVFBb in the absence or presence of 20 μg/ml C5 inhibitors. e C5 binding to C3b-coated beads (loaded with 1 μg/ml C3b-biotin) in absence or presence of 20 μg/ml C5 inhibitors, determined by flow cytometry. f C5 binding to pre-opsonized bacteria in absence or presence of 20 μg/ml C5 inhibitors (flow cytometry). c–f Data of three independent experiments, presented as means ± standard deviation (SD). Measures of statistical significance were determined by one-way ANOVA for the various inhibitors versus buffer control alone and displayed as: ns; *P <0.05; **P <0.01; ***P <0.005; and ****P <0.001