Electron Crystallography of Human Blood Coagulation Factor VIII Bound to Phospholipid Monolayers

Abstract

Coagulation factor VIII binds to negatively charged platelets prior to assembly with the serine protease, factor IXa, to form the factor X-activating enzyme (FXase) complex. The macromolecular organization of membrane-bound factor VIII has been studied by electron crystallography for the first time. For this purpose twodimensional crystals of human factor VIII were grown onto phosphatidylserine-containing phospholipid monolayers, under near to physiological conditions (pH and salt concentration). Electron crystallographic analysis revealed that the factor VIII molecules were organized as monomers onto the lipid layer, with unit cell dimensions: a ‫؍‬ 81.5Å, b ‫؍‬ 67.2 Å, ␥ ‫؍‬ 66.5°, P1 symmetry. Based on a homology-derived molecular model of the factor VIII (FVIII) A domains, the FVIII projection structure solved at 15-Å resolution presents the A1, A2, and A3 domain heterotrimer tilted approximately 65°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 organization of factor VIII allows the factor IXa protease and epidermal growth factor-like domain binding sites (localized in the A2 and A3 domains, respectively) to be situated at the appropriate position for the binding of factor IXa. The conformation of the lipid-bound FVIII is therefore very close to that for the activated factor VIIIa predicted in the FX-ase complex.Factor VIII (FVIII) 1 is an essential protein in blood coagulation (1). Deficiency in FVIII is responsible for hemophilia A (classic hemophilia), an X-chromosome-linked bleeding disorder (2). During coagulation, FVIII is proteolytically cleaved to an unstable active heterodimeric form (FVIIIa) by trace amounts of thrombin or factor Xa (FXa) (3). FVIIIa functions as a cofactor, responsible for the efficient activation of factor X (FX) by activated factor IX (FIXa) in the presence of negatively charged phospholipids (PL) and Ca 2ϩ ion (4). The assembly of the membrane-bound FX-ase complex (FX/FVIIIa/FIXa) results in the release of FXa, which associates with the cofactor factor Va (FVa) and prothrombin into the membrane-bound prothrombinase complex. The released thrombin further cleaves fibrinogen to insoluble fibrin, which stabilizes the growing clot and arrests bleeding (5).The binding of FVIIIa to FIXa enhances the catalytic reaction by at least 5 orders of magnitude (6). Although the molecular mechanism of this amplifying effect is not yet known, it seems to be strongly related to the formation of the FX-ase complex onto negatively charged membranes: increasing both the collision frequencies by restricting the reactants to a twodimensional surface, and the specificity of the FIXa active site (7,8).Human FVIII is a large plasma glycoprotein synthesized as a single-chain polypeptide with a predicted molecular mass of 264,763 Da, corresponding to the mature 2332 amino acid sequence, determined from translation of the cloned gene. With N-linked carbohydrate chains, the molecular mass ...

Factor IXa:Factor VIIIa Interaction

“…Several mutations in the proposed interface cause hemophilia A or B and are known to impair IXa:VIIIa interaction. Thus, our interface model is compatible with the existing biochemical as well as with the two-dimensional electron crystallography data of Stoylova et al (20). However, the three-dimensional cocrystal structure of the factor IXa protease domain and A2 subunit will be required to fully establish this view.…”

The factor VIII C1 domain contributes to platelet binding

“…28,29 This model indicates that C2 contacts the membrane, while C1 forms almost a right angle with C2, suggesting that C1 might not directly participate in membrane binding. In contrast, in a crystal structure of the homologous factor Vai protein (FVa, missing its A2 domain), C1 and C2 are oriented side by side, 30 such that both domains could interact with phospholipid membranes.…”

The crystal structure of activated protein C-inactivated bovine factor Va: Implications for cofactor function

“…Although a structural rearrangement due to APC inactivation cannot be completely ruled out, several pieces of evidence argue against this possibility. First, reconstructions of factor Va using electron microscopy (EM) depict a molecule extending Ϸ100 Å from the cell membrane (42), and these dimensions correlate well with the more recent 15-Å EM projection structure of factor VIIIa (43). In homology models of factors Va and VIIIa based on these EM data, a variety of domain orientations have been proposed (Fig.…”