INTERACTION OF THROMBOSPONDIN WITH PLATELET GLYCOPROTEINS GPI(A)-II(A) AND GPII(B)-III(A)

The interaction of human thrombospondin (TSP) with GPI(a)-II(a) and GPII(b)-III(a) was studied. The binding for both proteins became time-independent after 60 min. A 7-fold excess concentration of unlabelled GPI(a)-II(a) added either initially, or after time-dependent binding, resulted in a 50 % inhibition of GPI(a)-II(a) bound to TSP. GPI(a)-II(a) and GPII(b)-III(a) specifically bound TSP since: (a) the binding of GPII(b)-III(a) to TSP was dependent on the presence of 1 mM MgCl2 and 1 mM CaCl2, whereas binding of GPI(a)-II(a) was ion-independent. (b) The binding was saturable, with dissociation constants of 0.69 +/- 0.17 muM and 3.77 +/- 1.02 muM for GPI(a)-II(a) and GPII(b)-III(a) respectively. (c) GPII(b)-III(a) and GPI(a)-II(a) did not significantly bind to BSA. (d) GPII(b)-III(a) bound fibrinogen ion-specifically, whereas little or no binding of GPI(a)-II(a) was detectable. (e) Both GPII(b)-III(a) and GPI(a)-II(a) bound collagen in an ion-independent manner. (f) GPII(b)-III(a) did not compete with GPI(a)-II(a) for binding to TSP. (g) Binding of GPI(a)-II(a) to TSP was inhibited with anti-(GPI(a)-II(a)) (6FI), whereas mouse IgG and anti-(GPII(b)-III(a)) (AP-2) had no effect. (h) The interaction of GPI(a)-II(a) with TSP is 5.5-fold more favourable than that of GPII(b)-III(a) suggesting that GPI(a)-II(a) may be a preferred binding protein for TSP-mediated platelet adhesion.