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Biphasic regulation of mu‐opioid signaling to adenylyl cyclase by GTPase accelerating protein (GAP) activity of RGS7
Author(s) -
Talbot Jeffery Neil,
Roman David L,
Neubig Richard R,
Traynor John R
Publication year - 2007
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.21.5.a430
Subject(s) - g protein , adenylyl cyclase , gtpase activating protein , heterotrimeric g protein , camp dependent pathway , g protein coupled receptor , chemistry , microbiology and biotechnology , damgo , gtp' , gtp binding protein regulators , gs alpha subunit , biochemistry , signal transduction , biology , receptor , agonist , opioid receptor , enzyme
Agonist‐occupied G protein‐coupled receptors activate heterotrimeric G proteins by displacing GDP from Gα, which allows for formation of active GTP‐Gα and Gβγ signaling molecules. Regulators of G protein signaling (RGS) proteins negatively modulate G protein signaling by increasing the rate of Gα‐GTP hydrolysis. However, under conditions of enhanced G protein coupling, such as high receptor concentration, Gα‐GDP substrate is depleted making GTP hydrolysis the rate‐limiting step in G protein activation. In this environment, RGS proteins can augment receptor‐mediated G protein activation by replenishing Gα‐GDP (Huang et al., 2003). Using rat C6 glioma cells stably expressing the mu opioid receptor, we found that permeabilization with digitonin completely abolished inhibition of forskolin‐stimulated adenylyl cyclase (AC) by DAMGO, a mu opioid agonist. Addition of purified RGS domain of RGS7 (RGS7‐box) restored DAMGO‐mediated AC inhibition in cells expressing RGS‐sensitive but not RGS‐insensitive Gαo. Restoration of DAMGO activity occurred maximally at 0.32μM RGS7‐box but diminished at higher concentrations, suggesting a biphasic nature to this response. Furthermore, RGS7‐box increased DAMGO‐stimulated [ 35 S]GTPγS binding to Gαo by nearly two‐fold, consistent with an effect of RGS7 to increase Gα‐GDP. These data suggest that in digitonin‐permeabilized cells, RGS7 box has both positive and negative regulatory effects on G protein signaling to AC dependent on the extent of receptor/G protein coupling inherent in the system.