Translocation of cortactin (p80/85) to the actin-based cytoskeleton during thrombin receptor-mediated platelet...

Cortactin (p80/85) was discovered as a src kinase substrate and an actin filament binding protein. We investigated translocation of cortactin to the cytoskeleton during thrombin receptor-mediated platelet activation. Only a few percent of total cortactin (minor cortactin pool) translocates to the cytoskeleton as early as 5 s after platelet activation, while about 40% of total cortactin (major cortactin pool) is thereafter recovered in the cytoskeleton during platelet aggregation. Pretreatment of platelets with cytochalasin D suppresses completely this translocation, indicating that the translocation is dependent on actin polymerization. Inhibition of platelet aggregation by a tetrapeptide with the sequence RGDS, chelator of extracellular Ca2+, or a nonstirring condition results in marked suppression of translocation of the major cortactin pool. These results suggest that a minor cortactin pool translocates to the cytoskeleton independent of GPII-bIIIa (alpha IIb beta 3 integrin) engagement, and a major pool requires GPIIbIIIa-mediated signals into the cell for the translocation. Methyl 2,5-hydroxycinamate, a tyrosine kinase inhibitor, inhibits tyrosine phosphorylation of cortactin without affecting its translocation, indicating that tyrosine phosphorylation is not essential for the translocation. Morphological studies reveal that cortactin is colocalized with filamentous actin in aggregated platelets and that it is localized at the cell peripheries along actin filaments in spread platelets. Taking these together, we have demonstrated in this paper that the translocation of cortactin is associated with the reorganization of the actin-based cytoskeleton during platelet activation, particularly with platelet aggregation.