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Talin does not associate exclusively with alpha 2b beta 3 integrin in activated human platelets
G Escolar1, M Diaz-Ricart, J G White
1Hospital Clínico y Provincial, Barcelona, Spain.
Abstract:
Talin is a high-molecular-weight protein that may stabilize connections between cytoplasmic actin and the submembrane portion of glycoprotein IIb-IIIa (GPIIb-IIIa) (alpha 2b beta 3 integrin) in thrombin-stimulated human platelets. Using morphologic and electrophoretic techniques, we have examined the association of talin with the cytoskeleton of platelets activated by thrombin in the presence of fibrinogen-coated gold particles (Fgn/Au). Ultrastructural studies confirmed the presence of Fgn/Au firmly bound to the outside membranes of detergent-extracted platelets. Immunoblots of protein bands showed GPIIIa, but not talin, associated with cytoskeletons of activated platelets. Immunogold cytochemical techniques were performed on ultrathin cryosections of whole platelets to localize talin at the ultrastructural level. Studies were performed on normal platelets and platelets defective in GPIIb-IIIa (Glanzmann's thrombasthenia) and GPIb (Bernard-Soulier syndrome). Talin was randomly distributed in the cytoplasm of resting platelets. Activation resulted in binding of Fgn/Au to the surface membrane and redistribution of talin to the submembrane region. However, no definitive colocalization between the two markers was noted. Activated thrombasthenic platelets failed to bind Fgn/Au, but talin was localized to the submembrane location. After activation, talin was confined to the submembrane zone of Bernard-Soulier syndrome platelets. No definitive colocalization was observed between large clusters of Fgn/Au-occupied receptors and talin distributed in the submembrane region. GPIb and GPIIb-IIIa are not necessary for talin to localize in the submembrane region of activated cells. Talin does not redistribute exclusively with GPIIb-IIIa, and it may stabilize connections with other glycoproteins.
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