The C3bB-lufaxin–binding interface. (A) Surface representation of C3bB-lufaxin model colored with C3bB in light gray and lufaxin in light blue. Binding interface residues predicted by PISA are colored (both lufaxin and C3bB) in cyan for the Ba fragment of FB (CCP3 domain), red for the VWA domain of FB, and magenta for the CUB domain of C3b. Opening of the interface by opposite rotations of lufaxin and C3bB around the vertical axis with interaction regions colored as in the left panel. (B) Space filling representation showing details of the lufaxin surface with interface residues colored (as in panel A) (right) by C3bB interaction region. Residues forming hydrogen bond, salt bridge, or cation π interactions are labeled. (C) Details of lufaxin interactions with the Ba fragment and the VWA domains of FB. Boxed area of the ribbon structure is magnified, and interface residues are shown along with the ribbon diagram. Coloring: Ba fragment, cyan; VWA domain, red; and lufaxin, light blue. Backbone and side chain atoms are shown as sticks, with oxygen colored in red and nitrogen in blue. Hydrogen bonds and salt bridges are shown as red dashed lines. (D) Details of lufaxin interactions with the CUB domain of C3b in the C3bB-lufaxin complex. The magnified region shows lufaxin (light blue) and CUB (magenta) interactions highlighted as in panel C.