Simultaneous binding of C3bB and prothrombinase by lufaxin. Model of a hypothetical complex containing C3bB, lufaxin, and the prothrombinase complex (fVa and fXa) created by superimposing the SP domains of fXa in C3bB-lufaxin-fXa and prothrombinase (PDB ID 7TPP¹² with prothrombin coordinates removed). The left and right images are related by rotation around the vertical axis. C3bB is colored light gray and lufaxin light blue. fVa is colored yellow and fXa light green. Membrane contact points determined by Ruben et al¹² in fVa and fXa are colored red and the thioester-forming residues (Cys 988, Gln 991) in the TED domain of C3b are colored green in the right hand image. TED, thioester-containing domain.