A novel peptide (designed to bind to αIIbβ3) caused platelet aggregation and aggregation-independent secretion of the contents of α-granules in an αIIbβ3-dependent fashion. The agonist peptide induced secretion in the presence of prostaglandin E1. In cell-free assays, αIIbβ3 bound specifically to immobilized agonist peptide and the peptide enhanced the binding of fibrinogen to immobilized αIIbβ3. The agonist peptide apparently activates αIIbβ3 by directly inducing a conformational change in the receptor. This change activates the platelets and causes secretion in a manner independent of fibrinogen binding.

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