Thrombospondin acts via integrin-associated protein to activate the platelet integrin alpha(IIb)beta(3)

Integrin-associated protein (IAP or CD47) is a receptor for the cell/platelet-binding domain (CBD) of thrombospondin-1 (TS1), the most abundant protein of platelet alpha granules. Although it associates with alpha IIb beta 3, IAP has no known function in platelets. TS1, the CBD, and an IAP agonist peptide (4N1K) from the CBD of TS1 activate the platelet integrin alpha IIb beta 3, resulting in platelet spreading on immobilized fibrinogen, stimulation of platelet aggregation, and enhanced tyrosine phosphorylation of focal adhesion kinase. Furthermore, 4N1K peptide selectively stimulates the phosphorylation of LYN and SYK and their association with FAK. The phosphorylation of SYK is blocked by pertussis toxin, implicating a G(i)-like heterotrimeric G protein. IAP solublized from membranes of unstimulated platelets binds specifically to an affinity column of 4N1K peptide. Both alpha IIb and beta 3 integrin subunits and c-Src bind along with IAP. This complex of proteins is also detected with immunoprecipitation. Activation of platelets with the agonist peptide 4N1K results in the association of FAK with the IAP-alpha IIb beta 3 complex. Thus an important function of TS1 in platelets is that of a secreted costimulator of alpha IIb beta 3. whose unique properties result in its localization to the platelet surface and the fibrin clot.