Thrombospondin and fibrinogen bind serotonin-derivatized proteins on COAT-platelets.

Activation of platelets with 2 agonists, collagen and thrombin, reveals a subpopulation of cells referred to as COAT-platelets (collagen and thrombin activated). These cells are enriched in several membrane-bound, procoagulant proteins, including fibrinogen, thrombospondin, factor V, von Willebrand factor, and fibronectin. alpha-Granule proteins bound to COAT-platelets are derivatized with serotonin by a transglutaminase-mediated process, and the interaction of conjugated serotonins with unidentified serotonin binding sites on the platelet surface enhances retention of these proteins. We now demonstrate that both thrombospondin and fibrinogen provide the requisite serotonin binding sites. Thrombospondin and fibrinogen were identified using photoreactive cross-linking to an albumin-(serotonin)(6) conjugate during COAT-platelet production. We subsequently verified that biotin-albumin-(serotonin)(6) binds in vitro to thrombospondin, fibrinogen, and fibrinogen fragment D in a saturable manner. These data support a model for COAT-platelets where serotonin-derivatized procoagulant proteins interact with their respective receptors (eg, fibrinogen with glycoprotein IIb/IIIa or factor V with phosphatidylserine) as well as serotonin binding sites on fibrinogen and thrombospondin, resulting in a stable, multivalent complex on the cell surface.