Molecular Mechanism Controlling Acceleration Patterns of G-protein Signaling Induced by RGS Proteins.

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The recently discovered family of RGS (regulators of G protein signaling) proteins acts as GTPase activating proteins which bind to α subunits of heterotrimeric G proteins. We previously showed that a brain specific RGS, RGS8 speeds up the activation and deactivation kinetics of the G protein- coupled inward rectifier K+ channel (GIRK) upon receptor stimulation. Further, the acute desensitization of receptor-activated GIRK was observed with RGS8.1. We isolated a full-length rat cDNA of another brain specific RGS, RGS7. RGS7 cDNA encoded a protein of 477 amino acids with a long N-terminus containing 330 residues. RGS8 is a small RGS protein of 180 amino acids with a short N-terminus. When co-expressed with GIRK1/2 in Xenopus oocytes, RGS7 and RGS8 differentially accelerate G protein-mediated modulation of GIRK.RGS7 clearly accelerated activation of GIRK current similarly with RGS8 but the acceleration effect of deactivation was significantly weaker than that os RGS8. The acute desensiti … More zation of receptor-activated GIRK observed with RGS8 was not apparent with RGS7. These acceleration properties of RGS proteins may play important roles in the rapid regulation of neuronal excitability and the cellular responses to short-lived stimulations. The N-terminal domains of RGS proteins might function as a regulatory element for these diffenrential acceleration of G protein signaling.2. Functional roles of the N-terminal region of RGS8 in G-protein signaling were studied. The deletion of the N-terminal region of RGS8 (ΔNRGS8) resulted in a partial loss of the inhibitory function in pheromone response of yeasts, although Gα binding was not affected. To examine roles in subcellular distribution, we co-expressed two fusion proteins of RGS8-RFP and ΔNRGS8-GFP in DDT1MF2 cells. RGS8-RFP was highly concentrated in nuclei of unstimulated cells. Co-expression of constitutively active Gαo resulted in translocation of RGS8 protein to the plasma membrane. In contrast, ΔNRGS8-GFP was distributed diffusely through the cytoplasm in the presence or absence of active Gαo. When co-expressed with GIRK channels, ΔNRGS8 accelerated both turning-on and -off similar to RGS8. Acute desensitization of GIRK current observed in RGS8, however, was not induced by ΔNRGS8. Thus, we, for the first time, showed that the N-terminus of RGS8 contributes to the subcellular localization and to the desensitization of the G-protein-coupled response. Less