The protein kinase C (PKC) family of signal transducers are characterized by a dependence upon lipids for activity. Specifically, the classical (cPKCα, β and γ) and novel (nPKCδ, ϵ, η and θ) PKC isotypes display a physiological requirement for diacylglycerol for activity. This property of PKC has defined a now well established signalling pathway operating through receptors to phosphatidylinositol‐specific phospholipase C and hence via diacylglycerol (DAG)[and inositol (1, 4, 5) trisphosphate Ins (1, 4, 5) P 3/Ca 2+] to PKC (Figure 1). The operation of this pathway has been described in many cell types, and numerous reviews have covered this signalling paradigm (see Nishizuka, 1986; Hug et al., 1993; Dekker and Parker, 1994; Jaken, 1996).

image Figure 1 The classical pathway of PKC activation. The scheme illustrates the production of the immediate precursor lipid PtdIns (4, 5) P 2 from its parent lipid PtdIns. Various agonists are linked to the phospholipases (PtdIns‐PLC) that can cleave PtdIns (4, 5) P 2 to diacylglycerol (DAG) and the calcium mobilizer Ins (1, 4, 5) P 3. Calcium can affect the cPKC class by promoting membrane recruitment, but the key allosteric activator at the membrane for both cPKC and nPKC isotypes is DAG.