The platelet cytoskeleton stabilizes the interaction between alphaIIbbeta3 and its ligand and induces selective movements of ligand-occupied integrin.

Previously, we showed that a subpopulation of the major platelet integrin, alphaIIbbeta3, co-sediments from detergent lysates with talin and other membrane skeleton proteins. Once alphaIIbbeta3 has bound adhesive ligand in a platelet aggregate, the detergent-insoluble alphaIIbbeta3 redistributes (along with the detergent-insoluble membrane skeleton proteins and a variety of signaling molecules) to a fraction that contains cytoplasmic actin filaments. Concomitantly, certain signaling molecules are activated. The present study shows that, in intact platelets, alphaIIbbeta3 forms clusters when occupied by ligand and is selectively moved into the open canalicular system; alphaIIbbeta3 that has not bound ligand remains diffusely distributed at the periphery of the cell. When cytoplasmic actin filaments are depolymerized by cytochalasins, the ability of alphaIIbbeta3 to bind ligand is decreased, and the movement of ligand-occupied alphaIIbbeta3 is prevented. Together with the previous findings, these results suggest that (i) membrane skeleton-associated alphaIIbbeta3 is selectively induced to bind ligand in activated platelets, (ii) ligand-induced transmembrane signaling causes an altered association of membrane skeleton-associated alphaIIbbeta3 with the cytoplasmic component of the cytoskeleton, (iii) ligand-induced cytoskeletal reorganizations stabilize the interaction between ligand and integrin, and (iv) ligand-occupancy triggers cytoskeletal reorganizations that result in selective movements of occupied ligand.