(c) copyright 2004 David Siderovski & Francis Willard
Introduction
Heterotrimeric guanine-nucleotide binding proteins (G-proteins) are best known for transducing a wide array of extracellular signals received from heptahelical (7TM) cell-surface receptors, such as those initiated by hormones in the bloodstream, neurotransmitters across the synapse, and photons striking the retina. In the conventional model of heterotrimeric G-protein activation, the ligand/7TM receptor interaction catalyzes guanine nucleotide exchange on the Gbeta/gamma-complexed (and GDP-bound) Galpha subunit (Fig. 1). The Galpha·GTP and Gbeta/gamma subunits of the heterotrimer then dissociate and are thus free to propagate the signal forward via separate (and sometimes converging) interactions with adenylyl cyclases, phospholipase‑C isoforms, potassium and calcium ion channels, guanine-nucleotide exchange factors for the GTPase RhoA (“RhoGEFs”), and other “effector” enzymes [1, 2, 3]. The intrinsic GTP hydrolysis (GTPase) activity of the Galpha subunit resets the cycle by forming Galpha·GDP which has low affinity for effectors but high affinity for Gbeta/gamma. Thus, the inactive GDP-bound heterotrimer (Galpha/beta/gamma) is reformed, capable once again of interacting with activated receptor. Based on this cycle, the duration of heterotrimeric G-protein signaling is thought to be controlled by the lifetime of the Galpha subunit in its GTP-bound state. In 1996, we and others identified a superfamily of RGS (“regulator of G-protein signaling”) proteins that bind Galpha subunits via a hallmark ~120 amino-acid “RGS-box” domain [4, 5, 6, 7, 8, 9], dramatically accelerating their intrinsic GTPase activity [10, 11, 12], and thus attenuating heterotrimer-linked signaling (reviewed in [13, 14, 15]). The discovery of the RGS proteins and their GTPase-accelerating (or “GAP”) activity on Galpha subunits not only resolved apparent timing paradoxes between known 7TM-receptor-mediated physiological responses and the activity in vitro of the responsible G-proteins (e.g., [16]), but in addition, novel functional domains discovered within members of the RGS protein superfamily (Fig. 2) have helped to reveal hitherto unappreciated roles for Galpha subunits in cellular processes outside the realm of 7TM receptor signaling.
Figure 1. Standard model of the guanine nucleotide cycle governing 7TM receptor-mediated activation of heterotrimeric G protein-coupled signaling. The Gbeta/gamma heterodimer serves to couple Galpha to the receptor and also to inhibit its spontaneous release of GDP (i.e., acting as a guanine nucleotide dissociation inhibitor or “GDI” for Galpha [17, 18]). Ligand-occupied, 7TM cell-surface receptors stimulate signal onset by acting as guanine nucleotide exchange factors (GEFs) for Galpha subunits, facilitating GDP release, subsequent binding of GTP, and release of the Gbeta/gamma dimer. Both the GTP-bound Galpha and liberated Gbeta/gamma moieties are then able to modulate the activity of various enzymes, ion channels, and other effectors. Regulator of G-protein signaling (RGS) proteins stimulate signal termination by acting as GTPase-accelerating proteins (GAPs) for Galpha, dramatically enhancing their intrinsic rate of GTP hydrolysis.
Heterotrimeric guanine-nucleotide binding proteins (G-proteins) are best known for transducing a wide array of extracellular signals received from heptahelical (7TM) cell-surface receptors, such as those initiated by hormones in the bloodstream, neurotransmitters across the synapse, and photons striking the retina. In the conventional model of heterotrimeric G-protein activation, the ligand/7TM receptor interaction catalyzes guanine nucleotide exchange on the Gbeta/gamma-complexed (and GDP-bound) Galpha subunit (Fig. 1). The Galpha·GTP and Gbeta/gamma subunits of the heterotrimer then dissociate and are thus free to propagate the signal forward via separate (and sometimes converging) interactions with adenylyl cyclases, phospholipase‑C isoforms, potassium and calcium ion channels, guanine-nucleotide exchange factors for the GTPase RhoA (“RhoGEFs”), and other “effector” enzymes [1, 2, 3]. The intrinsic GTP hydrolysis (GTPase) activity of the Galpha subunit resets the cycle by forming Galpha·GDP which has low affinity for effectors but high affinity for Gbeta/gamma. Thus, the inactive GDP-bound heterotrimer (Galpha/beta/gamma) is reformed, capable once again of interacting with activated receptor. Based on this cycle, the duration of heterotrimeric G-protein signaling is thought to be controlled by the lifetime of the Galpha subunit in its GTP-bound state. In 1996, we and others identified a superfamily of RGS (“regulator of G-protein signaling”) proteins that bind Galpha subunits via a hallmark ~120 amino-acid “RGS-box” domain [4, 5, 6, 7, 8, 9], dramatically accelerating their intrinsic GTPase activity [10, 11, 12], and thus attenuating heterotrimer-linked signaling (reviewed in [13, 14, 15]). The discovery of the RGS proteins and their GTPase-accelerating (or “GAP”) activity on Galpha subunits not only resolved apparent timing paradoxes between known 7TM-receptor-mediated physiological responses and the activity in vitro of the responsible G-proteins (e.g., [16]), but in addition, novel functional domains discovered within members of the RGS protein superfamily (Fig. 2) have helped to reveal hitherto unappreciated roles for Galpha subunits in cellular processes outside the realm of 7TM receptor signaling.
Figure 1. Standard model of the guanine nucleotide cycle governing 7TM receptor-mediated activation of heterotrimeric G protein-coupled signaling. The Gbeta/gamma heterodimer serves to couple Galpha to the receptor and also to inhibit its spontaneous release of GDP (i.e., acting as a guanine nucleotide dissociation inhibitor or “GDI” for Galpha [17, 18]). Ligand-occupied, 7TM cell-surface receptors stimulate signal onset by acting as guanine nucleotide exchange factors (GEFs) for Galpha subunits, facilitating GDP release, subsequent binding of GTP, and release of the Gbeta/gamma dimer. Both the GTP-bound Galpha and liberated Gbeta/gamma moieties are then able to modulate the activity of various enzymes, ion channels, and other effectors. Regulator of G-protein signaling (RGS) proteins stimulate signal termination by acting as GTPase-accelerating proteins (GAPs) for Galpha, dramatically enhancing their intrinsic rate of GTP hydrolysis.