Possible Roles of Protein Kinase C in Neurotransmission

Abstract

Protein kinase C (PKC) is an ubiquitous Ca²⁺-activated phospholipiddependent protein kinase abundant in the central nervous system(1–4). This enzyme is activated by 1,2-diacylglycerol which transiently appears in the cell membrane, as a consequence of receptor-mediated and voltage-dependent hydrolysis of inositol phospholipids. The link of the phosphoinositide system to PKC seems to be that of a major second messenger system in the central nervous system and the possible involvement in significant steps in synaptic transmission have to be given attention. Until recently, PKC was thought to be a single entity, however, molecular cloning analysis has shown that there is a family of PKC-related genes(5–11). At least four subspecies of PKC (α, βI, βII and γ) are expresssed in the mammalian brain. Enzymatically, three major isozymes of PKC (types I, II and III) have been resolved from rat brains, using hydroxyapatite column chromatography(12). All subtypes were activated by diacylglycerol as well as tumor-promoting phorbol esters in the presence of Ca²⁺ and phosphatidylserine. They bound radioactive phorbol ester and showed similar kinetic properties, although the enzyme having the β-sequence was active at very low concentrations of Ca (8,13). The amino acid sequence of these three types has been identified by comparison with the enzyme separately expressed in the COS cells transfected by respective cDNAs-containing plasmids (14). Type I corresponds to the enzyme encoded with the γ-sequence. Type II is a mixture of βI- and βII-subspecies derived from alternative splicing of a single gene. Type III corresponds to α-subspecies. The αI-and βII-subspecies differ by only about 50 amino acid residues in the carboxyl-terminal regions, show similar kinetic properties and are not separable by conventional enzyme purification procedures. To characterize the tissue specificity of these subspecies, a specific antibody against each subspecies should be a pertinent tool. We present here the cell specific expression of each subspecies of PKC in the rat brain, determined using subspecies specific antibodies.