NMR Structural Analysis of Factor XI Apple 4 Domain.

Blood coagulation is achieved by two closely coordinated mechanisms: i) the contact factor, or intrinsic, pathway initiated by assembly of coagulation proteins on negatively charged surfaces, and ii) the extrinsic pathway initiated by exposure of tissue factor at the site of vascular injury. Although factor XI (FXI), a 160 KDa homodimeric plasma coagulation protein, can be activated either by FXIIa (generated by the contact factor pathway) or by thrombin (generated by the extrinsic or tissue factor pathway), recent evidence suggests that its unique dimeric structure is required for FXI-activation by thrombin on the platelet surface, leading to the initiation of the intrinsic pathway that is required for normal hemostasis. Each FXI monomer consists of an N-terminal heavy chain and a C-terminal trypsin-like catalytic light chain. The heavy chain consists of four homologous subunits called apple domains (designated A1 to A4). The A2 and A3 domains of one monomeric subunit bind to FIX, whereas the A3 domain of the other monomeric subunit binds to platelets. The A4 domain, which shares high (25–38%) sequence identity with other Apple domains, facilitates FXI dimer formation through an intermolecular disulfide bond at Cys-321. In the present study the rA4 domain was cloned and purified to determine its three-dimensional structure. Multidimensional heteronuclear NMR experiments were carried out using C¹³, N¹⁵, H² labeled samples. Chemical shifts of the C¹³, N¹⁵ and H¹ resonance of all the residues were assigned. Assignment of NOE cross peaks between inter- and intra-subunit amino acids is in progress. Preliminary results indicate that the monomeric structure of the A4 domain consists of six anti-parallel β-strands and an α-helix, stabilized by three cystine cross links. The orientations of charged residues and hydrophobic patches on different sides of the molecule may play important roles in the dimerization process of FXI.