Identification of an Inactivating Cleavage Site for Alpha-Thrombin on the Heavy Chain of Factor VA

Evrim Erdogan, Department of Chemistry, Cleveland State University, 2351 Euclid Avenue, Science and Research Center SR 370, Cleveland, Ohio 44115, USA.
Michael A. Bukys
Thomas Orfeo
Kenneth G. Mann
Michael Kalafatis

Abstract

Previous studies of factor (F)Va inactivation on human umbilical vein endothelial cells have shown that alpha-thrombin cleaves the heavy chain near the COOH-terminus to produce a M(r) 97,000 fragment containing the NH(2)-terminal portion of the heavy chain and a M(r) 8,000 peptide containing the rest of the molecule. The alpha-thrombin cleavage appeared to occur between amino acid residues 586 and 654 of FV. This region contains a consensus sequence for alpha-thrombin cleavage located at residues 640-644 (S-S-P-R-S). To test the hypothesis that alpha-thrombin cleaves the FVa heavy chain at Arg(643) and to evaluate the functional importance of this cleavage for FVa inactivation, site-directed mutagenesis was used to create recombinant FV molecules with mutations R(643) --> Q (FV(R643Q)) and R(643) --> A (FV(R643A)). All recombinant molecules were purified to homogeneity and assayed for activity following extended activation with alpha-thrombin. Under similar experimental conditions, appearance of the M(r) 97,000 heavy chain fragment in the plasma and wild-type FVa molecules correlated with partial loss of cofactor activity, while following extended incubation of FV(R643Q) and FV(R643A) with alpha-thrombin no cleavage of the heavy chain at Arg(643) was detected and no presence of the M(r) 97,000 heavy-chain fragment was noticed. Further, no loss in cofactor activity was observed using these mutant recombinant FVa molecules. Our data demonstrate that cleavage of FVa at Arg(643) by alpha-thrombin results in a partially inactive cofactor molecule and provides for an activated protein C (APC)-independent anticoagulant effect of alpha-thrombin.