Model of COAT-platelets.

This proposed model for COAT-platelets is not meant to be detailed or precise; there are currently too many unknown variables for a comprehensive model. The intent is to fit current observations into a feasible scheme. Basic observations include the following: COAT-platelets require serotonin derivatization of procoagulant proteins; serotonin binding sites are present on fibrinogen (Fbg) and thrombospondin (Tsp); traditional platelet receptors (eg, GP IIb/IIIa for Fbg) are occupied on COAT-platelets; and Fbg is bound to COAT-platelets with exceptional avidity. (A) A cross-section of the COAT-platelet membrane. Fbg, factor V (FV), and Tsp are bound to glycoprotein IIb/IIIa, phosphatidylserine (PS), and CD36, respectively. In addition, conjugated serotonin on FV interacts with serotonin binding sites on Fbg and/or Tsp, resulting in an enhanced stability for all 3 proteins. (B) A broader view of the cell surface showing some of the possible interactions present on COAT-platelets. For example, Fbg is not only binding GP IIb/IIIa and FV-5-HT, but it is also conjugated with 5-HT itself. Interactions between conjugated 5-HT on various proteins and fibrinogen or thrombospondin result in a 2-dimensional matrix with increased avidity for the cell surface. In the interest of simplicity, no membrane receptors are depicted in panel B.