A model for intracellular translocation of protein kinase C involving synergism between Ca²⁺ and phorbol esters

The activation of protein kinase C by diacylglycerol and by tumour promoters has implicated this enzyme in transmembrane signalling and in the regulation of the cell cycle^1,2. In vitro studies revealed that catalytic activity requires the presence of calcium and phos-pholipids with a preference f or phosphatidylserine³. Diacylglycerol and tumour promoters such as phorbol esters bind to the enzyme^4-7, leading to its activation while sharply increasing its affinity for Ca²⁺ and phospholipid. Addition of diacylglycerol analogues or phorbol esters to intact cells results in the phosphorylation of specific polypeptides^4,8-20. Several cellular processes, including hormone and neurotransmitter release and receptor down-regulation^1-4, are modulated by the activation of protein kinase C, while phorbol ester-induced stimulation of the enzyme in whole cells has been associated with its translocation from the cytoplasm to the plasma membrane^21,22. Moreover, the use of Ca²⁺ ionophores has revealed an apparent synergism between Ca²⁺ mobilization and protein kinase C activation². This synergism has recently also been found to apply to receptor down-regulation (ref. 23 and accompanying paper ²⁴). Here we describe a reconstitution system in which intracellular translocation of protein kinase C and the synergism between Ca²⁺ and enzyme activators can be studied. The results suggest a rationale for concomitant Ca²⁺ mobilization and diacylglycerol formation in response to some hormones, neuro-transmitters and growth factors.