• Lufaxin inhibits complement in assays of aHUS and PNH, as well as thrombin generation in plasma.

  • Lufaxin binds the C3 proconvertase of the alternative pathway and coagulation factor Xa at different sites allowing simultaneous inhibition.

Inhibitors of complement and coagulation are present in the saliva of a variety of blood feeding arthropods that transmit parasitic and viral pathogens. Here we describe the structure and mechanism of action of the sand fly salivary protein lufaxin, which inhibits formation of the central alternative C3 convertase (C3bBb) and inhibits coagulation factor Xa (fXa). Surface plasmon resonance experiments show that lufaxin stabilizes the binding of the serine protease factor B (FB) to C3b but does not detectably bind either C3b or FB alone. The crystal structure of the inhibitor reveals a novel all β-sheet fold containing two domains. A structure of the lufaxin-C3bB complex obtained by cryo-electron microscopy shows that lufaxin binds via its N-terminal domain at an interface containing elements of both C3b and FB. By occupying this spot, the inhibitor locks FB into a closed conformation where proteolytic activation of FB by FD cannot occur. C3bB-bound lufaxin binds fXa at a separate site in its C-terminal domain. In the cryo-EM structure of a C3bB-lufaxin-fXa complex the inhibitor binds both targets simultaneously and lufaxin inhibits fXa through substrate-like binding of a C-terminal peptide at the active site as well as other interactions in this region. Lufaxin inhibits complement activation in ex vivo models of atypical hemolytic uremic syndrome (aHUS) and paroxysmal nocturnal hemoglobinuria (PNH) as well as thrombin generation in plasma, providing rationale for the development of a bispecific inhibitor to treat complement-related diseases in which thrombosis is a prominent manifestation.

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