Thrombspondin Acts via Integrin-associated Protein to Activate the Platelet Integrin αIIbβ3*
- From the Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110
Abstract
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 α granules. Although it associates with αIIbβ3, 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 αIIbβ3, 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 Gi-like heterotrimeric G protein. IAP solublized from membranes of unstimulated platelets binds specifically to an affinity column of 4N1K peptide. Both αIIb and β3 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-αIIbβ3 complex. Thus an important function of TS1 in platelets is that of a secreted costimulator of αIIbβ3 whose unique properties result in its localization to the platelet surface and the fibrin clot.
Footnotes
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↵* This work was supported by a grant from Monsanto-Searle. A preliminary report of this data was presented at a meeting: The Thrombospondin Gene Family, June 19, 1996, University of Washington, Seattle, WA.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Biophysics, Box 8231, Washington University Medical School, 660 S. Euclid Ave., St. Louis, MO 63110. Tel.: 314-362-3348; Fax: 314-362-7183; E-mail: frazier{at}biochem.wustl.edu.
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↵1 The abbreviations used are: IAP, integrin-associated protein; CBD, cell binding domain; TS, thrombospondin; mAb, monoclonal antibody; HBS, Hepes-buffered saline; NGDA, nordihydroguaretic acid; FAK, focal adhesion kinase; PI, phosphatidylinositol; BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N′N′-tetraacetic acid; Ni-NTA, nickel nitrilotriacetic acid; ITAM, immunoreceptor tryosine-based activation motif.
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↵2 J. Chung, A.-G. Gao, and W. A. Frazier, unpublished results.
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- Received April 1, 1997.
