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 α-thrombin (αT; i.e. prolongs fibrin clotting time, tc, and inhibits αT-induced platelet 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 the 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 (platelet receptor glycoprotein Ibα) receptor, for binding to a continuous negative region on β2GpI structure, spanning domain IV and (part of) domain V, whereas 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 in the ensuing fibrin structure rather than by the 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 the GpIbα receptor, thus impairing platelet aggregation while enabling normal cleavage of fibrinogen and PC.
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Cover Image
The simultaneous binding of two antibodies to the same antigen (antibodies synergy or co-cooperativity) that elicit complement-mediated bactericidal activity is visualized with a model of a complex between Neisseria meningitides factor H binging protein (red cartoon) and monoclonal antibodies 12C1 (blue surface) and JAR5 (green surface). Monoclonal antibodies are schematically depicted with bars (colored light brown and yellow for heavy and light chains, respectively) in the background. Please see pp. 4699–4713 for more information. Picture generated and provided by Enrico Malito.
Molecular mapping of α-thrombin (αT)/β2-glycoprotein I (β2GpI) interaction reveals how β2GpI affects αT functions
Laura Acquasaliente, Daniele Peterle, Simone Tescari, Nicola Pozzi, Vittorio Pengo, Vincenzo De Filippis; Molecular mapping of α-thrombin (αT)/β2-glycoprotein I (β2GpI) interaction reveals how β2GpI affects αT functions. Biochem J 15 December 2016; 473 (24): 4629–4650. doi: https://doi.org/10.1042/BCJ20160603
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