Protein kinase C (PKC) consists of a family of serine/threonine protein kinases which are presumed to play a major role in signal transduction in response to the production of diacylglycerol (reviewed by Nishizuka, 1988; Parker et al. 1989). Originally defined as a kinase dependent on phospholipid and calcium for activity, PKC was found to be the major cellular receptor for the tumourpromoting phorbol esters, and the addition of either phorbol esters or diacylglycerol to the enzyme in vitro increases the affinity for phospholipid and calcium such that the kinase is fully active at physiological concentrations of these effectors. Subsequent isolation of cDNA clones encoding at least nine different PKC proteins from a minimum of seven genes (Parker et al. 1986; Coussens et al. 1986; Ono et al. 1987; Coussens et al. 1987; Ono et al. 1989a;...

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