STIMULATION OF PLATELET ACTIVATION AND AGGREGATION BY A CARBOXYL-TERMINAL PEPTIDE FROM THROMBOSPONDIN BINDING TO THE INTEGRIN-ASSOCIATED PROTEIN-RECEPTOR

Thrombospondin, a major secretory product of the alpha-granules of act ivated platelets, is a large trimeric glycoprotein that plays an impor tant role in platelet aggregation, On resting platelets, thrombospondi n binds to a single receptor in a cation-independent manner, but upon platelet activation it binds at least two further, distinct receptors that are both dependent upon divalent cations, Each of these receptors on the platelet surface binds to different regions of the thrombospon din molecule, and such binding may be responsible for the multifunctio nal role of thrombospondin in aggregation, We show here that a peptide from the carboxyl terminus of thrombospondin, RFYVVMWK, directly and specifically induces the activation and aggregation of washed human pl atelets from different donors at concentrations of 5-25 mu M. At lower concentrations the peptide synergizes with suboptimal concentrations of ADP to induce aggregation, Peptide affinity chromatography and immu noprecipitation with a monoclonal antibody were used to identify the r eceptor for the carboxyl terminal peptide as the integrin-associated p rotein, The integrin-associated protein remained bound to the RFYVVMWK -containing peptide column when washed with a scrambled peptide in the presence of 5 mM EDTA, indicating a divalent cation-independent assoc iation, It is suggested that integrin-associated protein is the primar y receptor for thrombospondin on the surface of resting platelets and is implicated in potentiating the platelet aggregation response.