FIG 9.8  The family of protein kinases C. Mammalian PKC comprises a family of nine distinct members subdivided, on the basis of their modes of activation, into three subfamilies: conventional (c), novel (n), and atypical (a). They are identified as PKC by the conserved domains C1, C2, C3, and C4. The protein is made up of regulatory and catalytic domains, connected by a hinge region. cPKC 1 and 2 are splice variants of the same gene. An alternative transcript exists for PKC which only encodes the catalytic domain and is referred to as PKM (not shown). We return to the matter of communication of atypical

PKCs with Par-6, p62, and Dsh on page 585.

The protein kinase C family

Molecular cloning has revealed a family of 9 genes giving rise to 11 distinct enzymes.45,46 Not all of these are activated by phorbol esters.47 They can be subdivided into three sub-families (see Table 9.1 and Figure 9.8). In addition, there are more distantly related enzymes such as PKC-related kinase (PRK, alternatively PKN),48 not considered here. The various isoforms are also referred to as isozymes, but this term is better reserved for an enzyme such as lactate dehydrogenase, since the isoforms of PKC have separate and characteristic substrates.

Table 9.1  PKC isoforms

DAG, diacylglycerol; PS, phosphatidylserine.