Properties of membrane-inserted Protein Kinase C

Protein kinase C (PKC) interacted with phospholipid vesicles in a calcium-dependent manner and produced two forms of membrane-associated PKC: a reversibly bound form and a membrane-inserted form. The two forms of PKC were isolated and compared with respect to enzyme stability, cofactor requirements, and phorbol ester binding ability. Membrane-inserted PKC was stable for several weeks in the presence of calcium chelators and could be rechromatographed on gel filtration columns in the presence of EGTA without dissociation of the enzyme from the membrane. The activity of membrane-inserted PKC was not significantly influenced by Ca²⁺, phospholipids, and/or PDBu. Partial dissociation of this PKC from phospholipid was achieved with Triton X-100, followed by dialysis to remove the detergent. The resulting free PKC appeared indistinguishable from original free PKC with respect to its cofactor requirements for activation (Ca²⁺, phospholipid, and phorbol esters), molecular weight, and phorbol 12,13-dibutyrate (PDBu) binding. The binding of PDBu to free and membrane-inserted PKC was measured under equilibrium conditions using gel filtration techniques. At 2.0 nM PDBu, free PKC bound PDBu with nearly 1:1 stoichiometry in the presence of Ca²⁺ and phospholipid. No PDBu binding to the free enzyme was observed in the absence of Ca²⁺. In contrast, membrane-inserted PKC bound PDBu in the presence or the absence of Ca₂₊; calcium did enhance the affinity of this interaction. These results indicated that neither Ca²⁺7 nor phorbol esters were needed to maintain the configuration of an active PKC complex once the enzyme was inserted into a membrane. Insertion into membranes could be the mechanism by which phorbol esters or diacylglycerols activate or induce long-term activation of PKC.