Figure 6.

A structure-based model for the inhibitory mechanism. A docking of hC3Nb2 onto the structures of C3bB (PDB entry 2XWJ) (A) and C3b:miniFH:FI (PDB entry 5O32) (B) using the hC3Nb2:C3c envelope is consistent with the interpretation that hC3Nb2 does not compete with either FB or FH. C, a superposition of the known structures at the MG3-MG4 interface suggests that the epitope of hC3Nb2 is conserved in C3 (PDB entries 2A73 and 2B39), C3b (PDB entry 5FO7), C3c (PDB entry 2A74), hC3Nb1:C3 (PDB entry 6RU5), and hC3Nb1:C3b (PDB entry 6EHG). D, mapping of hC3Nb2 onto native C3 (PDB entry 2A73) by comparison with C3c. E, a comparison between the model of the hC3Nb2:C3 complex in D and the general model of the convertase-substrate complexes implies that hC3Nb2 inhibits C3 cleavage by all convertases by preventing substrate recognition.