Differential coupling of RGS3s and RGS4 to GPCR-GIRK channel signaling complexes

\u27Regulators of G protein signaling\u27 (RGS proteins) modulate the G proteincycle by enhancing the GTPase activity of Ga subunits. These changesaccelerate the kinetics of ion channel modulation by Gai/o-coupled receptors(GPCRs) such as the G protein-gated inward rectifier K+ (GIRK/Kir3) channel. Myexperiments indicate that a single cerebellar granule (CG) neuron, a cell type thatendogenously expresses GIRK channels is able to express a wide variety ofRGS proteins. I selected two of them, which are widely expressed andtranscriptionally regulated during pathophysiologic conditions, to compare theirfunctional properties. I originally described the differential modulatory effects oftwo RGS proteins, the RGS3 short isoform (RGS3s) and RGS4, on muscarinicm2 and serotonin 1A receptor-coupled Kir3.1/Kir3.2a channels expressed inChinese hamster ovary (CHO-K1) cells. Both RGS3s and RGS4 acceleratedGIRK activation and deactivation current kinetics in a similar way. However, onlyRGS3s si gnificantly decreased the maximal GIRK current (Imax) elicited by ACh(~45% inhibition) and significantly increased the EC50 for both GPCRs. Thehypothesis that emerged from this initial study was that the distinct RGS4 Nterminaldomain mediated a direct coupling of RGS4 to GPCR-GIRK channelsignaling complexes that was not shared by RGS3s. To test this hypothesis, Iepitope-tagged several GPCRs, the Kir3.1 subunit, RGS3s, RGS4, and severaldeletion mutants and chimeras for co-immunoprecipitation experiments. Using anepitope-tagged degradation resistant RGS4 mutant RGS4(C2V), I detected coprecipitationof different GPCR-GIRK channel complexes with RGS4 but notRGS3s.The functional impact of RGS4 coupling to the GPCR-Kir3 channelcomplex versus uncoupled RGS3s was not apparent in recordings from CHO-K1cells presumably due to a high degree of RGS collision-coupling. Controlledexpression in Xenopus oocytes revealed a 30-fold greater potency for RGS4 inthe accelerating GIRK channel gating kinetics. In summary, these findings demonstrate that one of the ways for the cellto achieve signaling pathway specificity may be through selective coupling of thedifferent GPCR-effector-RGS protein complexes