Identification of Protein Kinase Cα as an Essential, but Not Sufficient, Cytosolic Factor for Ca²⁺-induced α- and Dense-core Granule Secretion in Platelets

Upon activation, platelets release many active substances. Here, we have analyzed the mechanism governing Ca²⁺-induced secretion of von Willebrand factor stored in α-granules and 5-hydroxytryptamine in dense-core granules in permeabilized human platelets. Both secretions were dependent on ATP and cytosol. An essential factor for both granule secretions was purified from rat brain cytosol and identified to be protein kinase Cα (PKCα) by partial amino acid sequencing. Purified PKCα efficiently stimulated both secretions in the presence of cytosol, whereas PKCα alone did not support the secretion of either type of granules, suggesting that PKCα is not a sufficient factor. Finally, in human platelet cytosol fractionated by a gel filtration column, the stimulatory activity for dense-core granule secretion paralleled with the concentration of PKC, suggesting that PKC could also be such a stimulatory factor in platelet cytosol. Thus, we identified PKCα as an essential, but not sufficient, cytosolic factor for the Ca²⁺-induced secretions of both α- and dense-core granules in platelets.