Thrombin activation of human platelets dissociates a complex containing gelsolin and actin from phosphatidylinositide-specific phospholipase C gamma(1)

We have examined the association of two cytoskeleton proteins, gelsolin and actin, with phosphatidylinositide-specific phospholipase C gamma(1) (PLC gamma(1)) in resting and thrombin-stimulated human platelets. In unstimulated platelets, gelsolin, actin and PLC gamma(1) were immunoprecipitated as a complex by a polyclonal antibody to PLC gamma(1). The association of gelsolin and actin was specific for PLC gamma(1) because immunoprecipitates of PLCs beta(2), beta(3), gamma(2) and delta(1), which are also expressed in human platelets, did not contain detectable gelsolin or actin. Activation with thrombin resulted in platelet aggregation and the dissociation of gelsolin and actin from PLC gamma(1). Inhibition of thrombin-induced platelet aggregation blocked the dissociation of gelsolin and actin from PLC gamma(1). After stimulation with thrombin, PLC gamma(1) activity in immunoprecipitates was increased 2-3-fold. This elevation in PLC gamma(1) activity in response to thrombin activation was not observed when platelet aggregation was blocked. Although PLC gamma(1) is tyrosine phosphorylated in response to many agonists, we could not detect, by Western analysis with anti-phosphotyrosine antibodies, tyrosine phosphorylation of PLC gamma(1) immunoprecipitated from thrombin-stimulated platelets. These results demonstrate that PLC gamma(1) is associated with gelsolin and actin in resting platelets, and that thrombin-induced platelet aggregation results in the dissociation of PLC gamma(1) from gelsolin and actin, and the stimulation of PLC gamma(1) activity.