Differential mechanisms of translocation of protein kinase C to plasma membranes in activated human neutrophils

Three classes of activators of human neutrophils that induce the intracellular translocation of protein kinase C from the cytosol to the particulate fraction were compared for their effects on the properties of the particulate (membrane-bound) enzyme. In cells stimulated with 10 ng/ml of phorbol-12-myristate-13-acetate (PMA) the particulate enzyme is almost fully active in the absence of added Ca²⁺ or phospholipids and this activity is not released by the Ca²⁺-chelator EDTA. In contrast, binding of protein kinase C to the particulate fraction in cells treated with the chemotactic factor f-Met-Leu-Phe (fMLF) or with the ionophore A-23187 plus Ca²⁺ is observed only when the cells are lysed in the presence of 1 mM Ca²⁺. With these stimuli the particulate enzyme retains a nearly absolute requirement for Ca²⁺ and phospholipids. Thus only the full intercalation of protein kinase C caused by PMA, which is resistant to removal by chelators stabilizes an active form of protein kinase C in the neutrophil membrane. In confirmation of this conclusion, in isolated plasma membranes loaded with partially purified protein kinase C by incubation with 5 μM Ca²⁺ further incubation with PMA, but not with fMLF, caused a significant fraction of the bound PKC to become resistant to removal by chelators, and to be nearly fully active in the absence of added activators.