Thrombin-Induced GPIb-IX Centralization on the Platelet Surface Requires Actin Assembly and Myosin I1 Activation

In resting platelets, the GPb-IX complex, the receptor for the von Willebrand factor (vWF), is linked to underlying actin filaments by actin-binding protein (ABP-280). Thrombin stimulation of human platelets leads to a decrease in the surface expression of the GPb-IX complex, which is redistributed from the platelet surface into the open canalicular system (OCS). Because the centralization of GPlb-IX is inhibited by cytochalasin, it is believed t o be linked t o actin cytoskeletal rearrangements that take place during platelet activation. We have further characterized the mechanism of GPb-IX centralization in platelets in suspension. Following thrombin stimulation, GPb-IX shifts from the membrane skeleton of the resting cell to the cytoskeleton of the activated cell in a reaction sensitive t o cytochalasin B. The cytoskeletal association of GPIb-IX involves ABP-280, as it correlates with the incorporation of ABP-280 into the activated cytoskeleton and because no dissociation of the ABP-2801 GPIb-IX complexes is detected after thrombin activation. However, the incorporation of GPlb-IX into the cytoskeleton is complete within 1 minute, whereas GPlb-IX centralization requires 5 to 10 minutes for completion. The movement of GPIb-IX to the cytoskeleton of activated platelets is therefore