The C553-C581 Disulfide of Human Factor XI Is Crucial for Its Activation and Is Targeted By Thiol Isomerase Pdir

The activation of the coagulation system is a key step in the process of thrombosis. Using genetically-modified mouse models and inhibitors, we and others have reported that thiol isomerases PDI, ERp57, ERp72, ERp5 and ERp46 promotes coagulation while TMX1 has an inhibitory effect. However, the mechanisms by which these enzymes regulate coagulation factors remain largely unknown. More efforts are needed to understand whether the thiol-disulfide exchange of coagulation factors contribute to their activation. There are 17 pairs of disulfide bonds on the factor XI (FXI). In this study we performed a comprehensive evaluation of the functions of all disulfide bonds on FXI. When FXI carrying single cysteine with the disruption of disulfide bonds were expressed in 293T cells, we found that the C122-C128 disulfide bond was critical for FXI synthesis process; the C2-C85, C28-C58 and C32-C38 of the A1 domain, the C92-C175 and C118-C147 of the A2 domain, the C182-C265, C208-C237 and C212-C218 of the A3 domain, the C273-C356, C299-C328 and C303-C309 of the A4 domain, as well as the C398-C414, C496-C563 and C527-C542 of the catalytic domain were necessary for the secretion of FXI. Although the ablation of C362-C482 and C553-C581 did not cause the defect of synthesis and secretion, disrupted C362-C482 and C553-C581 completely lost the activity in FXI-mediated thrombin generation and aPTT, suggesting that C362-C482 and C553-C581 are crucial functional disulfides of FXI. C362-C482 links heavy chain and light chain, which is necessary for calalytic domain getting access to the heavy chain containing the binding site for FIX. As shown by comparative molecular dynamics simulations, the C553-C581 breakage induced the significant conformational change of the loop 553CKGDS557, which probably interrupts the hydrogen bond of catalytic residues H413-S557 and the substrate binding affinity . To investigate whether FXI is targeted by thiol isomerases, FXI was incubated with PDI, ERp57, ERp72, ERp5, ERp46 and TMX1 as well as other thiol isomerases, followed by chromogenic substrate assay, we found that FXI activity was selectively inhibited by PDIr. As shown by 3-(N-maleimido-propionyl) biocytin (MPB) labeling, PDIr marked reduced the FXI disulfides. Furthermore, using thiol differential labeling and mass spectrometry, PDIr was found to reduce C553-C581 of FXI. In a laser-induced cremaster arteriole injury model, PDIr deficiency enhanced fibrin formation at the site of injury suggesting that PDIr inhibits coagulation in vivo, consistent with its inhibition and reduction of FXI. Taken together, this study identified the role of every single FXI disulfide in protein synthesis, secretion and enzymatic activity, and provides the new evidence showing that the C553-C581 of human factor XI is a novel crucial disulfide for its activation and is targeted by the thiol isomerase PDIr for coagulation.