Summary

The only known function of the 41 amino acid cleaved peptide (TR _1-41 )of the seven transmembrane domain thrombin receptor (PAR1) is toactivate platelets (as determined by aggregation, surface P-selectin,and fibrinogen binding to activated GPIIb-IIIa). We now demonstratethat TR _1-41 results in a concentration-dependent decrease in the plateletsurface expression of each component of the GPIb-IX-V complex, asdetermined by flow cytometry with a panel of monoclonal antibodies(including 6D1, directed against the von Willebrand factor binding siteon GPIb, and TM60, directed against the thrombin binding site onGPIb). TR _1-41 also decreased ristocetin-induced platelet agglutination.Immunoblotting after incubation of platelets with TR_1-41 revealedneither a loss of platelet GPIb nor increase in supernatant GPIb fragments.As demonstrated by immunoelectron microscopy, TR _1-41 resultedin a redistribution of GPIb, GPIX, and GPV from the platelet surfaceto the surface-connected canalicular system (SCCS). In summary,the cleaved peptide (TR _1-41 ) of PAR1 results in a redistribution of theplatelet surface GPIb-IX-V complex to the SCCS, thereby negativelyregulating the GPIbbinding sites for von Willebrand factor andthrombin.