Association of membrane and cytoplasmic proteins with the cytoskeleton in blood platelets

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Abstract
The association of membrane and cytoplasmic proteins with the cytoskeleton of resting and activated [human] 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, were 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. The Triton X-100 residue apparently contained the membrane-associated glycoprotein Ia, a 95,000-dalton protein, and 2 other acidic proteins of MW of 35,000-40,000, 1 labeled with 125INA and the other with azidofluorescein diacetate. The presence of part of these proteins in the Triton residue was dependent upon the mode of platelet activation. Glycoproteins IIb and III were embedded in the membrane lipid bilayer but sedimented with the Triton residue only after thrombin activation. Another protein with MW 70,000, which is highly labeled by 125INA in resting platelets, was found only in the Triton-soluble fraction.