Cleavage requirements for activation of factor V by factor Xa

Area of Science:

• Biochemistry
• Molecular Biology
• Hematology

Background:

• Coagulation factor V (FV) is a plasma protein precursor to factor Va (FVa), a crucial cofactor in thrombin generation.
• Activation of FV to FVa involves limited proteolysis by thrombin or factor Xa (FXa).
• Thrombin cleaves FV at Arg709, Arg1018, and Arg1545, while FXa cleaves at Arg1018 and potentially other sites.

Purpose of the Study:

• To elucidate the specific cleavage sites utilized by FXa during FV activation.
• To determine the contribution of each cleavage site (Arg709, Arg1018, Arg1545) to FVa procoagulant activity.
• To investigate the activation of FV by FXa, particularly in mutants resistant to thrombin.

Main Methods:

• Site-directed mutagenesis was employed to create FV variants with specific Arg residues (709, 1018, 1545) mutated to Gln or Ile.
• FV mutants were expressed in COS-1 cells and subsequently incubated with purified thrombin or FXa.
• Proteolytic cleavage patterns were analyzed by Western blotting, and functional FVa activity was quantified using a prothrombinase assay.

Main Results:

• FXa cleaves FV at all three thrombin sites (Arg709, Arg1018, Arg1545) and an additional site, Arg1765.
• Cleavage at Arg1018 by FXa alone was insufficient for full FVa activity; cleavage at both Arg709 and Arg1545 was necessary.
• FV mutated at all three thrombin sites was resistant to thrombin but exhibited increased FVa activity upon FXa treatment, linked to Arg1765 cleavage.

Conclusions:

• FXa activation of FV requires cleavage at Arg709 and Arg1545 for full procoagulant activity, similar to thrombin.
• FXa can activate FV via a novel site (Arg1765) in mutants lacking thrombin cleavage sites.
• This study clarifies the distinct roles of FXa and thrombin in FV activation and identifies a new activation pathway for FVa.