THE PLATELET CYTOSKELETON STABILIZES THE INTERACTION BETWEEN ALPHA(IIB)BETA(3) AND ITS LIGAND AND INDUCES SELECTIVE MOVEMENTS OF LIGAND-OCCUPIED INTEGRIN

Previously, we showed that a subpopulation of the major platelet integ rin, alpha(IIb)beta(3), co-sediments from detergent lysates with talin and other membrane skeleton proteins. Once alpha(IIb)beta(3) has boun d adhesive ligand in a platelet aggregate, the detergent-insoluble alp ha(IIb)beta(3) redistributes (along with the detergent-insoluble membr ane skeleton proteins and a variety of signaling molecules) to a fract ion that contains cytoplasmic actin filaments. Concomitantly, certain signaling molecules are activated, The present study shows that, in in tact platelets, alpha(IIb)beta(3) forms clusters when occupied by liga nd and is selectively moved into the open canalicular system; alpha(II b)beta(3) that has not bound ligand remains diffusely distributed at t he periphery of the cell. When cytoplasmic actin filaments are depolym erized by cytochalasins, the ability of alpha(IIb)beta(3) to bind liga nd is decreased, and the movement of ligand-occupied alpha(IIb)beta(3) is prevented. Together with the previous findings, these results sugg est that (i) membrane skeleton-associated alpha(IIb)beta(3) is selecti vely induced to bind ligand in activated platelets, (ii) ligand-induce d transmembrane signaling causes an altered association of membrane sk eleton-associated alpha(IIb)beta(3) with the cytoplasmic component of the cytoskeleton, (iii) ligand-induced cytoskeletal reorganizations st abilize the interaction between ligand and integrin, and (iv) ligand-o ccupancy triggers cytoskeletal reorganizations that result in selectiv e movements of occupied ligand.