Electron crystallography of human blood coagulation factor VIII bound to phospholipid monolayers

Coagulation factor WI binds to negatively charged platelets prior to assemb ly with the serine protease, factor Ma, to form the factor X-activating enz yme (FX-ase) complex. The macromolecular organization of membrane-bound fac tor VIII has been studied by electron crystallography for the first time. F or this purpose two-dimensional crystals of human factor VIII were grown on to phosphatidylserine-containing phospholipid monolayers, under near to phy siological conditions (pH and salt concentration), Electron crystallographi c analysis revealed that the factor VIII molecules were organized as monome rs onto the lipid layer,with unit cell dimensions: a = 81.5 Angstrom, b = 6 7.2 Angstrom, gamma = 66.5 degrees, P1 symmetry. Based on a homology-derive d molecular model of the factor VIII (FVIII) A domains, the FVIII projectio n structure solved at 15-Angstrom resolution presents the A1, A2, and A3 do main heterotrimer tilted approximately 65 degrees relative to the membrane plane. The A1 domain is projecting on top of the A3, C1, and C2 domains and with the A2 domain protruding partially between A1 and A3. This organizati on of factor VIII allows the factor IXa protease and epidermal growth facto r-like domain binding sites (localized in the A2 and A3 domains, respective ly) to be situated at the appropriate position for the binding of factor IX a The conformation of the lipid-bound FVIII is therefore very close to that for the activated factor VIIIa predicted in the FX-ase complex.