The structure of a heterotrimeric G protein reveals the mechanism of the nucleotide-dependent engagement of the alpha and beta gamma subunits that regulates their interaction with receptor and effector molecules. The interaction involves two distinct interfaces and dramatically alters the conformation of the alpha but not of the beta gamma subunits. The location of the known sites for post-translational modification and receptor coupling suggest a plausible orientation with respect to the membrane surface and an activated heptahelical receptor.

Heterotrimeric G proteins have a crucial role as molecular switches in signal transduction pathways mediated by G-protein-coupled receptors. Extracellular stimuli activate these receptors, which then catalyse GTP-GDP exchange on the G protein alpha-subunit. The complex series of interactions and conformational changes that connect agonist binding to G protein activation raise various interesting questions about the structure, biomechanics, kinetics and specificity of signal transduction across the plasma membrane.

A large number of hormones, neurotransmitters, chemokines, local mediators, and sensory stimuli exert their effects on cells and organisms by binding to G protein-coupled receptors. More than a thousand such receptors are known, and more are being discovered all the time. Heterotrimeric G proteins transduce ligand binding to these receptors into intracellular responses, which underlie physiological responses of tissues and organisms. G proteins play important roles in determining the specificity and temporal characteristics of the cellular responses to signals. They are made up of ␣, ␤, and ␥ subunits, and although there are many gene products encoding each subunit (20 ␣, 6 ␤, and 12 ␥ gene products are known), four main classes of G proteins can be distinguished: G s , which activates adenylyl cyclase; G i , which inhibits adenylyl cyclase; G q , which activates phospholipase C; and G 12 and G 13 , of unknown function.G proteins are inactive in the GDP-bound, heterotrimeric state and are activated by receptor-catalyzed guanine nucleotide exchange resulting in GTP binding to the ␣ subunit. GTP binding leads to dissociation of G␣⅐GTP from G␤␥ subunits and activation of downstream effectors by both G␣⅐GTP and free G␤␥ subunits. G protein deactivation is rate-limiting for turnoff of the cellular response and occurs when the G␣ subunit hydrolyzes GTP to GDP. The recent resolution of crystal structures of heterotrimeric G proteins in inactive and active conformations provides a structural framework for understanding their role as conformational switches in signaling pathways. As more and more novel pathways that use G proteins emerge, recognition of the diversity of regulatory mechanisms of G protein signaling is also increasing. The recent progress in the structure, mechanisms, and regulation of G protein signaling pathways is the subject of this review. Because of space considerations, I will concentrate mainly on recent studies; readers are directed to a number of excellent reviews that cover earlier studies. G Protein StructureG␣ subunits contain two domains, a domain involved in binding and hydrolyzing GTP (the G domain) that is structurally identical to the superfamily of GTPases including small G proteins and elongation factors (1) and a unique helical domain that buries the GTP in the core of the protein (2, 3) (Fig. 1). The ␤ subunit of heterotrimeric G proteins has a 7-membered ␤-propeller structure based on its 7 WD-40 repeats (4 -6). The ␥ subunit interacts with ␤ through an N-terminal coiled coil and then all along the base of ␤, making extensive contacts (Fig. 1). The ␤ and ␥ subunits form a functional unit that is not dissociable except by denaturation. G protein activation by receptors leads to GTP binding on the G␣ subunit. The structural nature of the GTP-mediated switch on the G␣ subunit is a change in conformation of three flexible regions designated Switches I, II, and III to a well ordered, GTP-bound activated conformation with lowered affinity for G␤␥ (7) (Fig. 1). This leads to increased affin...

The 2.2 A crystal structure of activated rod transducin, Gt alpha.GTP gamma S, shows the bound GTP gamma S molecule occluded deep in a cleft between a domain structurally homologous to small GTPases and a helical domain unique to heterotrimeric G proteins. The structure, when combined with biochemical and genetic studies, suggests: how an activated receptor might open this cleft to allow nucleotide exchange; a mechanism for GTP-induced changes in effector and receptor binding surfaces; and a mechanism for GTPase activity not evident from previous data.

The 1.8 A crystal structure of transducin alpha.GDP, when compared to that of the activated complex with GTP-gamma S, reveals the nature of the conformational changes that occur on activation of a heterotrimeric G-protein alpha-subunit. Structural changes initiated by direct contacts with the terminal phosphate of GTP propagate to regions that have been implicated in effector activation. The changes are distinct from those observed in other members of the GTPase superfamily.

Aluminium fluoride (AIF-4) activates members of the heterotrimeric G-protein (G alpha beta gamma) family by binding to inactive G alpha.GDP near the site occupied by the gamma-phosphate in G alpha.GTP (ref. 3). Here we describe the crystal structure of transducin alpha.GDP activated with aluminium fluoride (Gt alpha.GDP.AIF-4.H2O) at 1.7 A, a resolution sufficient to establish the coordination geometry of the bound aluminium fluoride as well as the extensive network of direct and water-mediated interactions that stabilize it. These observations are derived from three independent representations in the asymmetric unit, eliminating any chance of drawing conclusions based on stereochemistry imposed by crystal packing. Surprisingly, aluminium fluoride activates G alpha.GDP by binding with a geometry resembling a pentavalent intermediate for GTP hydrolysis. The stabilizing interactions involve not only residues that interact with the gamma-phosphate in Gt alpha.GTP gamma S, but also conserved residues for GTPase activity. Thus the Gt alpha.GDP.AIF-4.H2O structure provides new insight into the mechanism of GTP hydrolysis.

In multicellular organisms from Caenorhabditis elegans to Homo sapiens, the maintenance of homeostasis is dependent on the continual flow and processing of information through a complex network of cells. Moreover, in order for the organism to respond to an ever-changing environment, intercellular signals must be transduced, amplified, and ultimately converted to the appropriate physiological response. The resolution of the molecular events underlying signal response and integration forms the basis of the signal transduction field of research. An evolutionarily highly conserved group of molecules known as heterotrimeric guanine nucleotide-binding proteins (G proteins) are key determinants of the specificity and temporal characteristics of many signaling processes and are the topic of this review. Numerous hormones, neurotransmitters, chemokines, local mediators, and sensory stimuli exert their effects on cells by binding to heptahelical membrane receptors coupled to heterotrimeric G proteins. These highly specialized transducers can modulate the activity of multiple signaling pathways leading to diverse biological responses. In vivo, specific combinations of G alpha- and G beta gamma-subunits are likely required for connecting individual receptors to signaling pathways. The structural determinants of receptor-G protein-effector specificity are not completely understood and, in addition to involving interaction domains of these primary acting proteins, also require the participation of scaffolding and regulatory proteins.

In spite of the recognition that striatal D(2) receptors are critical determinants in a variety of psychomotor disorders, the cellular mechanisms by which these receptors shape neuronal activity have remained a mystery. The studies presented here reveal that D(2) receptor stimulation in enkephalin-expressing medium spiny neurons suppresses transmembrane Ca(2+) currents through L-type Ca(2+) channels, resulting in diminished excitability. This modulation is mediated by G(beta)(gamma) activation of phospholipase C, mobilization of intracellular Ca(2+) stores, and activation of the calcium-dependent phosphatase calcineurin. In addition to providing a unifying mechanism to explain the apparently divergent effects of D(2) receptors in striatal medium spiny neurons, this novel signaling linkage provides a foundation for understanding how this pivotal receptor shapes striatal excitability and gene expression.