Protein Targeting Interactions in Signalling Processes

Introduction Protein kinase C (PKC) is a family of phospholipiddependent serine-threonine kinases that regulate cell growth and differentiation (reviewed in [ 1 4 ) . PKC is also the major intracellular receptor for tumour-promoting phorbol esters [ 1-31. Molecular cloning and biochemical separation of the individual gene products has led to a classification system for the PKC isoenzymes based on domain homology and biochemical properties. Conventional PKCs (a, PI, PI,, y ) are Ca2+-dependent phorbol ester receptor/kinases, whereas the novel PKCs (6, E , 7, 0) are Ca2+-independent phorbol ester receptorlkinases [ 1-31. The atypical PKCs ( 5 and A) are neither Ca2 + -dependent nor phorbol ester receptors [ 1-31. Most cells and tissues express several PKCs, suggesting that the isoenzymes do not have overlapping functions. The activation of PKC 7 by cholesterol sulphate and PKC 5 by phosphatidylinositol trisphosphate in vitro suggests the possibility that activation of individual PKCs by different second messenger pathways in vivo may result in isoenzyme-specific responses [4,5]. The fact that several agonists including platelet-derived growth factor [6,7], bombesin [7], endothelin-l[8], phenylephrine [8] and oleic acid [9] can selectively activate novel PKCs 6 and/or E in vivo supports the hypothesis that isoenzyme-specific responses may be mediated in part by different signal transduction pathways. In principle, the phosphorylation of different substrate proteins by individual isoenzymes should lead to distinct cellular responses. At present, little is known about isoenzyme substrate specificity in vivo. In vitro, phosphorylation of 'real' physiological substrates by different PKCs is similar [lo]. The fact that isoenzyme-specific substrate interactions are difficult to demonstrate in assays in vitro suggests that isoenzyme specificity may be dictated by interactions with other proteins and/or other