Identification of protein kinase C alpha as an essential, but not sufficient, cytosolic factor for Ca2+-induced alpha- and dense-core granule secretion in platelets

Upon activation, platelets release many active substances. Here, we have an alyzed the mechanism governing Ca2+-induced secretion of von Willebrand fac tor stored in alpha -granules and 5-hydroxytryptamine in dense-core granule s in permeabilized human platelets. Both secretions were dependent on ATP a nd cytosol. An essential factor for both granule secretions was purified fr om rat brain cytosol and identified to be protein kinase C alpha (PKC alpha ) by partial amino acid sequencing. Purified PKC alpha efficiently stimulat ed both secretions in the presence of cytosol, whereas PKC alpha alone did not support the secretion of either type of granules, suggesting that PKC a lpha is not a sufficient factor. Finally, in human platelet cytosol fractio nated by a gel filtration column, the stimulatory activity for dense-core g ranule secretion paralleled with the concentration of PKC, suggesting that PKC could also be such a stimulatory factor in platelet cytosol. Thus, we i dentified PKC alpha as an essential, but not sufficient, cytosolic factor f or the Ca2+-induced secretions of both alpha- and dense-core granules in pl atelets.