Structural studies of Gαq signaling and regulation
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Gαq signaling is implicated in a number of physiological processes that include platelet activation, cardiovascular development and smooth muscle function. Historically, Gαq is known to function by activating its effector, phospholipase Cβ. Desensitization of Gαq signaling is mediated by G-protein coupled receptor kinases (GRK) such as GRK2 that phosphorylates the activated receptor and also sequesters activated Gαq and Gβγ subunits. Our crystal structure of Gαq-GRK2-Gβγ complex shows that Gαq forms effector-like interactions with the regulator of G-protein signaling (RGS) homology domain of GRK2 involving the classic effector-binding site of Gα subunits, raising the question if GRK2 can itself be a Gáq effector and initiate its own signaling cascade. In the structure, Gα and Gβγ subunits are completely dissociated from one another and the orientation of activated Gαq with respect to the predicted cell membrane is drastically different from its position in the inactive Gαβγ heterotrimer. Recent studies have identified a novel Gαq effector, p63RhoGEF that activates RhoA. Our crystal structure of the Gαq-p63RhoGEF-RhoA complex reveals that Gαq interacts with both the Dbl homology (DH) and pleckstrin homology (PH) domains of p63RhoGEF with its C-terminal helix and its effector-binding site, respectively. The structure predicts that Gαq relieves auto-inhibition of the catalytic DH domain by the PH domain. We show that Gαq activates p63RhoGEF-related family members, Trio and Kalirin, revealing several conduits by which RhoA is activated in response to Gq-coupled receptors. The Gαq effector-site interaction with p63RhoGEF/GRK2 does not overlap with the Gαq-binding site of RGS2/RGS4 that function as GTPase activating proteins (GAPs). This suggests that activated G proteins, effectors, RGS proteins, and activated receptors can form high-order complexes at the cell membrane. We confirmed the formation of RGS-Gαq-effector complexes and our results suggest that signaling pathways initiated by GRK2 and p63RhoGEF are regulated by RGS proteins via both allosteric and GAP mechanisms. Our structural studies of Gαq signaling provide insight into protein-protein interactions that induce profound physiological changes. Understanding such protein interfaces is a key step towards structure-based drug design that can be targeted to treat diseases concerned with impaired Gαq signaling.