Like PKA and CaMK II, PKC is a serine/threonine kinase that is a direct target of G protein regulatory pathways. Four classical PKC isoforms (a, ßI, ßII, and y) are activated by the binding of diacylglycerol (DAG) and Ca2+ to two stretches of conserved residues located within the kinase regulatory domain (Tanaka and Nishizuka 1994). Numerous channels have been implicated as targets of PKC phosphorylation, but only rarely have the specific enzyme isoforms involved in their modulation been identified. This is largely due to the current lack of isoform-specific pharmacological activators or blockers of these kinases. In rat cortical pyramidal neurons as well as hippocampal CA3 cells, the stimulation of PKC by phorbol esters depresses the inhibitory effects of glutamate on N-type Ca2+ channels, thereby enhancing synaptic transmitter release (Swartz etal. 1993). The effect can be blocked by the inclusion of a PKC-specific inhibitor (PKC 19-36) in the pipette. As outlined earlier, glutamate acts through metabotropic receptors to reduce Ca2+ currents via a ßy-dependent channel inhibition. The role of PKC in reversing the Gßy-mediated effect was examined more closely by Zamponi et al. (1997), who found that certain peptides comprising the cytoplasmic I-II linker region of the a1B Ca2+ channel subunit acted as substrates for PKC phosphorylation in vitro. Gßy subunits normally bind to this linker region on the native channel to inhibit its activity. The perfusion of non-phosphorylated peptides into cells that express recombinant N-type Ca2+ channel subunits was shown to interfere with Gßy-mediated channel inhibition (Zamponi et al. 1997). However, prior phosphorylation of these peptides by PKC decreased their abilities to interrupt this process, suggesting that kinase modulation antagonizes the binding of ßy subunits to the peptides and to the corresponding regions of the channel.
L-type Ca2+ channels found in the heart are both positively and negatively regulated by PKC and often exhibit a biphasic response consisting of an initial current increase followed by a subsequent delayed decrease (Lacerda et al. 1988; Tseng and Boyden 1991; Satoh 1992). When the cardiac a1c Ca2+ channel is expressed in Xenopus oocytes, currents are inhibited by phorbol ester-induced PKC stimulation after a brief initial increase (Bourinet et al. 1992). The kinase recognition motif on this channel has been mapped to a short stretch of amino acids within the amino terminus of the protein which is unique among members of the Cav 1.2 subfamily. Mutations of two critical serine residues in this region completely abrogate the regulatory effects of the kinase, providing clear evidence that the observed decline in Ca2+ current involves a direct action of PKC on the channel (McHugh et al. 2000).
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