PROTEOLYSIS OF THROMBOSPONDIN DURING CATHEPSIN-G-INDUCED PLATELET-AGGREGATION - FUNCTIONAL-ROLE OF THE 165-KDA CARBOXY-TERMINAL FRAGMENT

The serine-proteinase cathepsin G (CG) is a potent agonist of platelet aggregation inducing the release and surface expression of alpha-gran ule adhesive proteins such as fibrinogen (Fg) and thrombospondin-l (TS P-1), Because Fg and TSP-1 are potential substrates for the enzymatic activity of CG, we investigated the fate of these proteins during CG-i nduced platelet aggregation using an immunoblot technique, Only a smal l proportion of secreted Fg was proteolyzed by CG and platelet aggrega tion was efficiently inhibited by anti-fibrinogen Fab fragments, In co ntrast, TSP-1 was extensively proteolyzed on aggregated platelets rele asing in the milieu a fragment with M(r) approximate to 28 000, corres ponding to the amino-terminal heparin-binding domain (HBD), Several an tibodies, directed against the cell-associated carboxy-terminal TSP-lf fragment (M(r) approximate to 165 000) impaired the formation of stab le macroaggregates, indicating that this fragment may contribute to pl atelet aggregation in the absence of the HBD.