Annexin V Relocates to the Platelet Cytoskeleton Upon Activation and Binds to a Specific Isoform of Actin

We have previously reported that stimulation of platelets causes a relocation of annexin V to the cytoplasmic side of the plasma membrane where it associates with actin. This study examined the association of annexin V with the platelet cytoskeleton and its binding to actin, following both physiological activation with thrombin and Ca2+ -ionophore activation. The time-dependence of annexin V incorporation into the detergent-extracted cytoskeleton following activation with thrombin was also measured. Although calcium from the intracellular stores was enough to relocate intracellular annexin V to the cytoskeleton, this relocation was further enhanced by influx of extracellular calcium. The association of annexin V with the cytoskeleton was found to be unaffected by the action of cytochalasin E, however, annexin V was solubilized when DNase I was used to depolymerize the membrane cytoskeleton, and spontaneously re-associated with the actin filaments when re-polymerization was induced in vitro. Using a bifunctional crosslinking reagent we have identified an 85-kDa complex in both membrane and cytoskeleton fractions containing annexin V and actin. Direct binding to actin filaments was only observed in high [Ca2+], however, inclusion of an extract from thrombin-stimulated platelets lowered the [Ca2+] requirement for the binding of annexin V to F-actin to physiological levels. We also show that GST-annexin V mimics the physiological binding of annexin V to membranes, and that this GST-annexin V binds directly to a specific isoform of actin. Immunoprecipitation using antibodies against annexin V copurify annexin V and gamma- but not beta-actin from activated platelets. This is the first report of a possible preferential binding of annexin V to a specific isoform of actin, namely gamma-actin. The results of this study suggest a model in which annexin V that relocates to the plasma membrane and binds to gamma-actin in an activation-dependent manner forms a strong association with the platelet cytoskeleton.