Journal of Biological Chemistry vol:283 issue:33 pages:22573-22581
Inactivation of factor Va (FVa) by activated protein C (APC) is a key reaction in the down- regulation of thrombin formation. FVa inactivation by APC is correlated with a loss of FXa cofactor activity as a result of three proteolytic cleavages in the FVa heavy chain at Arg(306), Arg(506), and Arg(679). Recently, we have shown that heparin specifically inhibits the APC- mediated cleavage at Arg(506) and stimulates cleavage at Arg(306). Three- dimensional molecular models of APC docked at the Arg(306) and Arg(506) cleavage sites in FVa have identified several FVa amino acids that may be important for FVa inactivation by APC in the absence and presence of heparin. Mutagenesis of Lys(320), Arg(321), and Arg(400) to Ala resulted in an increased inactivation rate by APC at Arg(306), which indicates the importance of these residues in the FVa- APC interaction. No heparin- mediated stimulation of Arg(306) cleavage was observed for these mutants, and stimulation by protein S was similar to that of wild type FVa. With this, we have now demonstrated that a cluster of basic residues in FVa comprising Lys(320), Arg(321), and Arg(400) is required for the heparin-mediated stimulation of cleavage at Arg(306) by APC. Furthermore, mutations that were introduced near the Arg(506) cleavage site had a significant but modest effect on the rate of APC- catalyzed FVa inactivation, suggesting an extended interaction surface between the FVa Arg(506) site and APC.