The venom of the Australian snake Pseudonaja textilis comprises powerful prothrombinactivators consisting of factor X (v-ptFX)- and factor V-like proteins. While all vertebrateliver-expressed... Show moreThe venom of the Australian snake Pseudonaja textilis comprises powerful prothrombinactivators consisting of factor X (v-ptFX)- and factor V-like proteins. While all vertebrateliver-expressed factor X (FX) homologs, including that of P. textilis, comprise anactivation peptide of approximately 45 to 65 residues, the activation peptide of vptFXis significantly shortened to 27 residues. In this study, we demonstrate thatexchanging the human FX activation peptide for the snake venom ortholog impedesproteolytic cleavage by the intrinsic factor VIIIa–factor IXa tenase complex. Furthermore,our findings indicate that the human FX activation peptide comprises anessential binding site for the intrinsic tenase complex. Conversely, incorporation ofFX into the extrinsic tissue factor–factor VIIa tenase complex is completely dependenton exosite-mediated interactions. Remarkably, the shortened activation peptide allowsfor factor V-dependent prothrombin conversion while in the zymogen state. Thisindicates that the active site of FX molecules comprising the v-ptFX activation peptidepartially matures upon assembly into a premature prothrombinase complex. Takentogether, the shortened activation peptide is one of the remarkable characteristics of vptFXthat has been modified from its original form, thereby transforming FX into apowerful procoagulant protein. Moreover, these results shed new light on thestructural requirements for serine protease activation and indicate that catalyticactivity can be obtained without formation of the characteristic Ile16–Asp194 saltbridge via modification of the activation peptide. Show less
