Functional studies of single-site variants in the calmodulin-binding domain of RC3/neurogranin in Xenopus oocytes.

Single-site variants in the calmodulin-binding domain of RC3/neurogranin were heterologously expressed in Xenopus oocytes to examine their effects on serotonin-evoked currents. RC3 variants serine36 -->alanine (Ser36-->Ala), serine36-->glycine (Ser36-->Gly), and phenylalanine37-->tryptophan (Phe37-->Trp), which bind calmodulin but are deficient in protein kinase C (PKC) phosphorylation, display serotonin-evoked Ca(2+)-dependent Cl- currents in oocytes similar to control oocytes. A serine36-->aspartate (Ser36-->Asp) variant, which does not bind calmodulin and mimics the PKC-phosphorylated state of RC3, significantly enhances serotonin-evoked currents in a manner similar to wild-type. The results suggest that RC3 not only regulates the availability of free calmodulin in a dendritic spine but also, when phosphorylated, independently stimulates G-protein coupled second messenger pathways that generate inositol 1,4,5-trisphosphate (IP3), diacylglycerol (DAG) and intracellular Ca2+.