Summary

Integrin αIibβ3 plays a pivotal role in platelet aggregation bybinding to fibrinogen. The β3 cytoplasmic domain of αIibβ3interacts with cytoskeletal and signaling proteins and is cleavedby µ-calpain, a calcium regulated cysteine protease. In thepresent study, we have investigated in more detail the cleavageof the β3 cytoplasmic domain during platelet aggregationinduced by thrombin,TRAP-1 and TRAP-4. Our data show thatβ3 is cleaved in all three cases.The time course of β3 cleavageand the amount of cleaved β3 depends on the way platelets areactivated and on the complete activation of µ-calpain, with amaximum of 90% of cleaved β3 obtained when thrombin is used. Furthermore, our results also show that the cleavedαIibβ3 is mainly distributed in the Triton soluble fraction, indicatingits inability to bind to the cytoskeleton. Interestingly, inthe absence of GPIb or following inhibition of thrombin bindingto GPIb , there is a reduction in the thrombin-induced calciumflux, β3 cleavage and µ-calpain activation. These resultssuggest that cleavage of the β3 cytoplasmic domain by µ-calpainmight be an important step regulating the link between thecytoskeleton and αIibβ3 during platelet aggregation, and thatGPIb could function as a cofactor for the complete activationof platelets by thrombin.