Molecular Mapping of α -Thrombin( α T)/ β 2-Glycoprotein I(β2GpI) Interaction Reveals How β2GpI Affects α T Functions

: β2-Glycoprotein I (β2GpI) is the major autoantigen in the antiphospholipid syndrome, a thrombotic autoimmune disease. Nonetheless, the physiological role of β2GpI is still unclear. In a recent work, we have shown that β2GpI selectively inhibits the procoagulant functions of human a-thrombin (αT) (i.e. prolongs fibrin clotting time, tc, and inhibits αT-induced platelets aggregation) without affecting the unique anticoagulant activity of the protease, i.e. the proteolytic generation of the anticoagulant protein C (PC) from the PC zymogen, which interacts with αT exclusively at the protease catalytic site. Here we used several different biochemical/biophysical techniques and molecular probes for mapping the binding sites in αT-β2GpI complex. Our results indicate that αT exploits the highly electropositive exosite-II, which is also responsible for anchoring αT on the platelet GpIbα receptor, for binding to a continuous negative region on β2GpI structure, spanning domain IV and (part of) domain V, while the protease active site and exosite-I (i.e. the fibrinogen binding site) remain accessible for substrate/ligand binding. Furthermore, we provided evidence that the apparent increase in tc, previously observed with β2GpI, is more likely caused by alteration of the ensuing fibrin structure rather than by inhibition of fibrinogen hydrolysis. Finally, we produced a theoretical docking model of αT-β2GpI interaction, which was in agreement with the experimental results. Altogether, these findings help to understand how β2GpI affects αT interactions and suggest that β2GpI may function as a scavenger of αT for binding to GpIbα receptor, thus impairing platelets aggregation while enabling normal cleavage of fibrinogen and PC.