Functional consequences of expression of the neuron-specific, protein kinase C substrate RC3 (neurogranin) in Xenopus oocytes.

RC3 (neurogranin) is a neuron-specific substrate of protein kinase C (PKC) that accumulates predominantly in dendritic spines of forebrain neurons and undergoes long-term potentiation (LTP)-associated increases in PKC-phosphorylation in hippocampal slices. Here the hypothesis that RC3 functions by modulating the IP3/DAG second messenger pathway after its phosphorylation by DAG-activated PKC was tested by heterologous expression in Xenopus oocytes. Acetylcholine-evoked inward chloride (Cl-) currents, dependent on both IP3 release and intracellular calcium (Ca2+), were 2- to 3-fold higher in RC3-injected oocytes than in uninjected control oocytes. RC3-oocytes did not exhibit enhanced currents when preincubated with the protein kinase inhibitor H-7 or when a glycine residue was substituted for serine, the PKC phosphorylation site of RC3. Activation of endogenous oocyte PKC by phorbol esters generated inward Cl- currents in RC3 oocytes but not in control oocytes. RC3-dependent Cl- currents were also elicited by phorbol ester in Ca(2+)-free media. We propose that PKC-phosphorylated RC3 is capable of enhancing the mobilization of intracellular Ca2+ in Xenopus oocytes and, by inference, may play a role in Ca2+ homeostasis in dendrites of forebrain neurons.