Autoproteolysis or plasmin-mediated cleavage of factor Xaα exposes a plasminogen binding site and inhibits coagulation

Abstract

Blood coagulation factor Xa (FXa) has recently been shown to function as a plasminogen receptor in the presence of procoagulant phospholipid (phosphatidylserine; PS) and Ca2+. In the current work, the possible effect of autoproteolytic and plasmin-mediated cleavage of FXa on complex formation with plasminogen was investigated. 125I-plasminogen binding to derivatives of FXa electrotransferred to polyvinylidene difluoride revealed that the autoproteolytic conversion of FXaα to FXaβ was required for the expression of a plasminogen binding site. In the presence of PS and Ca2+, plasmin was shown to convert FXaα to a FXaβ-like species at least 3 orders of magnitude faster than the autoproteolytic mechanism. This also resulted in the exposure of a plasminogen binding site. Further processing by plasmin generated a fragment (33 kDa) due to cleavage at Gly331 in the FXa heavy chain. Production of this species enhanced apparent plasminogen binding compared with FXaβ and resulted in the loss of FXa amidolytic and clotting activity. In the absence of either PS or Ca2+, the plasmin-mediated fragmentation of FXaα was altered to include a FXaβ-like molecule and a species (40 kDa) with intact β-heavy chain disulfide linked to a COOH-terminal fragment of the light chain starting at Tyr44. Neither of these products was observed to interact with plasminogen. The 40-kDa species had amidolytic activity comparable with FXaα but inhibited clotting activity. Cumulatively the data provide the first evidence for a functional difference between the FXa subforms and suggest a mechanism where autoproteolysis and plasmin-mediated cleavage modulate the function of FXaα from a procoagulant enzyme to a profibrinolytic plasminogen receptor.