Association of membrane and cytoplasmic proteins with the cytoskeleton in blood platelets.
The association of membrane and cytoplasmic proteins with the cytoskeleton of resting and activated platelets was studied. Glycoproteins were identified by labeling with 125I-labeled lectins (concanavalin A, wheat germ agglutinin, and Lens culinaris). Polypeptides, which are embedded in the lipid bilayer, have been identified by their photolabeling with the lipid-soluble reagent 5-[125I]iodonaphthyl 1-azide (125INA). Cytoplasmic proteins were identified by their photolabeling with the intracellular probe azidofluorescein diacetate. Results indicate that the Triton X-100 residue contains the membrane-associated glycoprotein Ia, a 95 000-dalton protein, and two other acidic proteins of molecular weights of 35 000-40 000, one labeled with 125INA and the other with azidofluorescein diacetate. The presence of part of these proteins in the Triton residue is dependent upon the mode of platelet activation. Glycoproteins IIb and III are embedded in the membrane lipid bilayer but sedimented with the Triton residue only after thrombin activation. Another protein with Mr 70 000, which is highly labeled by 125INA in resting platelets, is found only in the Triton-soluble fraction.