Figure 1.

(a) Domain organization of the homologous proteins C2 and factor B of human complement. (b) Cartoon representation of classical and alternative pathway C3 convertase formation. Abbreviations used: CCP, complement control protein; vWFA, von Willebrand factor type A domain; SP, serine protease domain; C2b and Ba, the N-terminal fragments of C2 and factor B that are made up of CCP modules and released upon formation of the convertases C4bC2a and C3bBb, respectively; C2a and Bb, the C-terminal fragments of C2 and factor B that remain associated with cofactors C4b and C3b in the formation of the C3 convertases C4bC2a and C3bBb, respectively; C4b and C3b, the active fragments of C4 and C3, respectively.