Cryo-EM structure of coagulation factor V short.

Coagulation factor V (fV) is the inactive precursor of fVa, an essential component of the prothrombinase complex required for rapid activation of prothrombin in the penultimate step of the coagulation cascade. In addition, fV regulates the tissue factor pathway inhibitor α (TFPIα) and protein C pathways that inhibit the coagulation response. A recent cryogenic electron microscopy (cryo-EM) structure of fV has revealed the architecture of its A1-A2-B-A3-C1-C2 assembly but left the mechanism that keeps fV in its inactive state unresolved due to intrinsic disorder in the B domain. A splice variant of fV, fV short, carries a large deletion of the B domain that produces constitutive fVa-like activity and unmasks epitopes for binding of TFPIα. The cryo-EM structure of fV short was solved at 3.2 Å resolution and reveals the arrangement of the entire A1-A2-B-A3-C1-C2 assembly for the first time. The shorter B domain stretches across the entire width of the protein, making contacts with the A1, A2 and A3 domains but suspended over the C1 and C2 domains. In the portion distal to the splice site, several hydrophobic clusters and acidic residues provide a likely binding site for the basic C-terminal end of TFPIα. In fV, these epitopes may bind intramolecularly the basic region of the B domain. The cryo-EM structure reported in this study advances our understanding of the mechanism that keeps fV in its inactive state, provides new targets for mutagenesis and facilitates future structural analysis of fV short in complex with TFPIα, protein S and fXa.