The structure and function of murine factor V and its inactivation by protein C.

Factor V (FV) is a central regulator of hemostasis, serving both as a critical cofactor for the prothrombinase activity of factor Xa and the target for proteolytic inactivation by the anticoagulant, activated protein C (APC). To examine the evolutionary conservation of FV procoagulant activity and functional inactivation by APC, we cloned and sequenced the coding region of murine FV cDNA and generated recombinant wild-type and mutant murine FV proteins. The murine FV cDNA encodes a 2,183-amino acid protein. Sequence comparison shows that the A1-A3 and C1-C2 domains of FV are highly conserved, demonstrating greater than 84% sequence identity between murine and human, and 60% overall amino acid identity among human, bovine, and murine FV sequences. In contrast, only 35% identity among all three species is observed for the poorly conserved B domain. The arginines at all thrombin cleavage sites and the R305 and R504 APC cleavage sites (corresponding to amino acid residues R306 and R506 in human FV) are invariant in all three species. Point mutants were generated to substitute glutamine at R305, R504, or both (R305/R504). Wild-type and all three mutant FV recombinant proteins show equivalent FV procoagulant activity. Single mutations at R305 or R504 result in partial resistance of FV to APC inactivation, whereas recombinant murine FV carrying both mutations (R305Q/R504Q) is nearly completely APC resistant. Thus, the structure and function of FV and its interaction with APC are highly conserved across mammalian species.