Receptor-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. 

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

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. 

Simons, P. C., Biggs, S. M., Waller, A., Foutz, T., Cimino, D. F., Guo, Q., Neubig, R. R., Tang, W. J., Prossnitz, E., and Sklar, L. A. (2004). Real-time analysis of ternary complex on particles Direct evidence for partial agonism at the agonist-receptor-G-protein complex assembly step of signal transduction. /. Biol. Chem. 279, 13514-13521. 

Fykse EM, Li C, and Stidhof TC (1995) Phosphorylation of rabphilin-3A by Ca2+/calmodulin-and cAMP-dependent protein kinases in vitro. J Neurosci 15 2385-95 Gales C, Van Durm JJ, Schaak S et al (2006) Probing the activation-promoted structural rearrangements in preassembled receptor-G protein complexes. Nat Stmct Mol Biol 13 778-86 Gamper N, Reznikov V, Yamada Y et al (2004) Phosphatidylinositol [correction] 4,5-bisphosphate signals underlie receptor-specific Gq/11-mediated modulation of N-type Ca2+ channels. J Neurosci 24 10980-92... 

Numerous studies have suggested that PLA2 activity changes in neural disorders [12-17], cancer [18-21], inflammatory disorders [8,22-24], and parturition [25,26], In addition, lysophosphatidylcholine (LPC) causes demyelination [27], Recent studies suggested that LPC, converted from oxidized low-density lipoprotein (ox-LDL), induces endothelium-dependent vasoconstriction [28] and abolishes the formation of the receptor G-protein complex [29],... 

On the basis of these results eombined with experimental data, a model of the receptor-G-protein complex was constructed. The reeeptor was orientated so that il3 and il4. fomied by palmitoylation of two eysteines in the C-terminal, were in eontact distance of Gpy. This left the first eluster as the primary interaetion site for ill and ill. The analysis of these results in the light of reeent work on GPCR dimerisation has led to some interesting results (see below). 

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