Figure 1.

The alternative pathway and negative stain EM analysis. A, the AP can be initiated by tick-over generated C3(H₂O) or from C3b generated within the classical or lectin activation pathways. Both C3b or C3(H₂O) can form the proconvertase with FB. C3b can be degraded to iC3b by FI assisted by a cofactor. The (P) signifies that the AP convertases may bind and be stabilized by properdin. B, negative stain EM 2D classes of the complex between hC3Nb1 and C3b. The TE domain adopts multiple alternative positions as compared with the C3b MG1–8 domains. Scale bar, 100 Å. C, 3D reconstruction of negative stain EM particles of the C3b–hC3Nb1 complex with the crystal structure of C3b (PDB entry 5FO7) fitted to the envelope. Excess density corresponding to the Nb is marked by asterisks.