Factor V: a platelet cytoskeletal associated protein

Platelet cytoskeletons prepared from thrombin activated platelets contain specifically associated Factor Va. We postulate that Factor Va associates with the cytoskeleton through receptors present on the platelet surface. The following evidence supports this hypothesis. (a) Prior secretion of Factor Va is necessary for the association of Factor Va with the cytoskeleton. (b) Reagents that inactivate Factor Va on the surface of the platelet such as EDTA and proteolytic enzymes also inactivate the Factor Va on the cytoskeleton. (c) The platelet Factor Xa binding sites (i.e. Factor Va) are quantitatively retained on the platelet cytoskeleton. (d) Platelet cytoskeletons prepared from platelets that are thought to be deficient in Factor Va binding sites contain 25 percent of Factor Va activity of normal platelet cytoskeletons and platelet cytoskeletons prepared from platelets deficient in Factor Va contain no Factor Va activity but regain control levels of Factor Va only when Factor Va is added to the platelets prior to the preparation of cytoskeletons.