G-protein complex

FIGURE 3.13 Major components of the cubic ternary complex model [25-27]. The major difference between this model and the extended ternary complex model is the potential for formation of the [ARjG] complex and the [RiG] complex, both receptor/ G-protein complexes that do not induce dissociation of G-protein subunits and subsequent response. Efficacy terms in this model are a, y, and 5. 

Cubic ternary complex model, a molecular model (J. Their. Biol 178, 151-167, 1996a 178, 169-182, 1996b 181, 381-397, 1996c) describing the coexistence of two receptor states that can interact with both G-proteins and ligands. The receptor/G-protein complexes may or may not produce a physiological response see Chapter 3.11. 

In general, the receptor-G-proteins complexes exchange bound GDP for GTP. In turn, the two, smaller subunits of the G-protein components of these complexes are released and the receptor protein dissociates. The remaining G-protein GTP complex then complexes with and activates a specific enzyme. It is very significant to note that G-proteins therefore have at least three specific binding sites (a) for nucleotides, (b) for a receptor protein, and (c) an effector protein. 

Law and Reisine [73] reported that the cloned 8 receptor physically associated with G0. They solubilized the 8 receptor with a mild detergent which allowed solubilized 8 receptors to remain associated with G proteins. They then showed that antisera directed against G0 co-immunoprecipitated 8 receptor/G protein complexes. 

Figure 4.13. Model of peptide initiation of mast secretion. Insertion of the hydrophobic region of the peptide into the lipid bilayer properly orients the basic (+) groups at the N-terminus for binding to negatively charged membrane components. As a result, there is activation of the G protein complex with the subsequent generation of inositol triphosphate (IP ) and diacylglycerol (DAG). These intermediates then stimulate the mobilization of cellular Ca and possibly the transient influx of extracellular Ca as well as the activation ofprotein kinase C. As a consequence, the level of intracellular free ionized Ca is maintained at an elevated state. The end result is the exocytotic extrusion of secretory granules.

Several different types of G-protein complex have been described. In general terms, if a cell becomes activated by ligand binding, the G-protein complex is said to be... 

Thus far, the discussion of G-proteins and effector enzymes has assumed that a ligand has engaged with its surface receptor. There is however, an important example of an alternative mechanism to activate an effector without the direct involvement of G-protein complex. NO is a local hormone, a neurotransmitter and part of the cell s armoury of oxidizing agents called free radicals. 

Raymond, J. R. (1994) Hereditary and acquired defects in signaling through the hormone-receptor-G protein complex. Am. J. Physiol. 266, 163-174. 

All muscarinic receptors are members of the seven transmembrane domain, G protein-coupled receptors, and they are structurally and functionally unrelated to nicotinic ACh receptors. Activation of muscarinic receptors by an agonist triggers the release of an intracellular G-protein complex that can specifically activate one or more signal transduction pathways. Fortunately, the cellular responses elicited by odd- versus even-numbered receptor subtypes can be conveniently distinguished. Activation of Ml, M3, and M5 receptors produces an inosine triphosphate (IP3) mediated release of intracellular calcium, the release of diacylglyc-erol (which can activate protein kinase C), and stimulation of adenylyl cyclase. These receptors are primarily responsible for activating calcium-dependent responses, such as secretion by glands and the contraction of smooth muscle. 

Cholera is a condition caused by a protein exotoxin produced by the bacterium vibrio cholerae. This protein toxin consists of six subunits one A subunit and five B subunits. The B subunits are responsible for the binding of the toxin to cAMP-functioning cells in small bowel of the intestines. The A subunit penetrates the cell and has catalytic activity which attaches the ADP portion of naturally occurring NAD (nicotine-adenosine dinucleotide) to the G-protein complex thereby inhibiting its GTPase activity. This deprives the complex of its "off-switch" for cAMP formation. The effect is the uncontrolled... 

Gales C, Van Durm JJ, Schaak S, Pontier S, Percherancier Y, Audet M, Paris H, Bouvier M (2006) Probing the activation-promoted structural rearrangements in preassembled receptor-G protein complexes. Nat Struct Mol Biol 13(9) 778—786... 

C. Current Approaches to Modeling the Receptor-G Protein Complex 77... 

A great deal of structural information about G proteins is known from x-ray crystallographic studies, providing insight into GTP-mediated conformational changes in Ga, subunit interactions with effector proteins, and the mechanism of GTP hydrolysis. By contrast, relatively little structural information is known about the interaction between the receptor and G protein and how this interaction leads to GDP release. After an overview of the structure of heptahelical receptors and heterotrimeric G proteins, this chapter will discuss the current models of the receptor-G protein complex and proposed mechanisms for receptor-catalyzed nucleotide exchange. 

Computational techniques have allowed investigators to use this extensive amount of biochemical and biophysical information to develop the available structural information, including the inactive structure of rhodopsin (T eller et al., 2001), the GDP-bound Gt heterotrimer (Lambright et al., 1994), and NMR structures of the C-termini of Gat and Gy (Kisselev and Downs, 2003 Kisselev et al., 1998), into molecular models of the receptor-G protein complex (Ciarkowski et al., 2005 Fotiadis et al., 2004 Slusarz and Ciarkowski, 2004). Since the C-terminus of Ga, a critical receptor-binding site, is absent in both structures of G protein heterotrimers (Lambright... 

Fig. 5. Models of the receptor-G protein complex. Two representations of receptor-G protein complexes are shown. The R Ga(0) fly complex created by manually docking the G protein onto an activated receptor model based on the rhodopsin crystal structure (1GZM) (left panel). Sites on Got that cross-link to residue S240 (cyan sphere) on the receptor are highlighted (cyan). This sort of data will be critical for improving models of the receptor-G protein complex, as it provides constraints for the location of IC 3 relatively to Ga. In this model, the nucleotide-binding pocket is some 30 A away from the receptor-binding surface. The model of an Ro Ga (O) [jy-com pIex is based on coordinates generously provided by K. Palczewski (published in Fotiadis et al, 2004).

Despite the fundamental importance of receptor-catalyzed G protein activation in cellular signaling, relatively little is known about this process as compared to the other regulatory events in the G protein cycle. Crystal structures of inactive rhodopsin and G protein heterotrimers provide the structural context for the meaningful interpretation of the results of the numerous biochemical and biophysical studies described in the preceding sections. Enough data has been accumulated to begin to develop rudimentary models of the receptor-G protein complex that meet some of... 

Particularly important information will come from additional distance constraints between the receptor and G protein, such as novel cross-links or interprobe measurements from fluorescence or electron paramagnetic resonance spectroscopy. Initially, relatively few long-range distance constraints would be necessary to define the relative orientation of the receptor and G protein, which is a fundamental but unanswered question about the complex. Once the proper orientation has been established experimentally, the model will suggest additional distance measurements that will be necessary to pin down the receptor s intracellular loops onto the surface of the G protein. Proceeding in such an iterative fashion should provide the experimental evidence that is critically important in refining the current models of the receptor—G protein complex. 

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