Receptor-G protein interactions and

Heterologous desensitisation refers to the desensitisation of the response to one agonist by the application of a different agonist. For example, desensitisation of a response to adrenaline by application of 5-HT is mediated by protein kinase A or protein kinase C because these kinases can phosphorylate receptors which are not occupied by agonist. Phosphorylation disrupts the receptor-G-protein interaction and induces the binding of specific proteins, arrestins which enhance receptors internalisation via clathrin-coated pits. Thus desensitisation of G-protein-coupled receptors results in a decrease in the number of functional receptors on the cell surface. 

Fluoride effects on the thyroid can be observed on many different levels. Of particular importance relating to G-protein activation is the ability of AlFx to clone the role of thyroid-stimulating hormone (TSH). Allgeier with coworkers [113] demonstrated that TSH led to the activation of two forms of Gs (Gs short and Gs long) as well as of Gq and Gil, demonstrating that signaling pathways induced by TSH already bifurcate in the course of the receptor-G-protein interaction and concluded that TSH activation is mediated by Gq/11 and Gs, respectively. 

The interaction between the receptor and the G-protein is transient and rapidly reversible. This is indicated, for example, by the fact that a single light-activated rhodopsin molecule may activate 500 to 1000 transducin molecules during its 1 to 3 sec lifetime. Hence, the interaction should, in the endpoint, be governed by the normal laws of chemical interaction and expressible in terms of association and dissociation rate constants and binding affinity. The question then arises as to whether the affinity of different receptors for different G-proteins varies. That is, is there specificity in receptor-G-protein coupling, and, if so, what determines this ... 

Clapham, D. E. and Neer, E., G-protein Py subunits, Anna. Rev. Pharmacol. Toxicol., 37, 167-203, 1997. Gudermann, T., Kalkbrenner, F., and Schultz, G., Diversity and selectivity of receptor-G-protein interaction, Annu. Rev. Pharmacol. Toxicol., 36, 429-459, 1996. 

Muscarinic receptor activation causes inhibition of adenylyl cyclase, stimulation of phospholipase C and regulation of ion channels. Many types of neuron and effector cell respond to muscarinic receptor stimulation. Despite the diversity of responses that ensue, the initial event that follows ligand binding to the muscarinic receptor is, in all cases, the interaction of the receptor with a G protein. Depending on the nature of the G protein and the available effectors, the receptor-G-protein interaction can initiate any of several early biochemical events. Common responses elicited by muscarinic receptor occupation are inhibition of adenylyl cyclase, stimulation of phos-phoinositide hydrolysis and regulation of potassium or other ion channels [47] (Fig. 11-10). The particular receptor subtypes eliciting those responses are discussed below. (See also Chs 20 and 21.)... 

Jin, H., Zastawny, R., George, S. R., and O Dowd, B. F. (1997) Elimination of palmitoylation sites in the human dopamine receptor does not affect receptor-G protein interaction. Eur. J. Pharmacol. 324, 109-116. 

P) of surface membranes to Ca2+, Na+, and K+. The resulting depolarization or hyperpolarization of the sensory cell (the signal) is passed through relay neurons to sensory centers in the brain. In the best-studied cases, desensitization includes phosphorylation of the receptor and binding of a protein (arrestin) that interrupts receptor-G protein interactions. VR is the vasopressin receptor other receptor and G protein abbreviations are as used in earlier illustrations. 

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. 

Hedin, K. E., Duerson, K., and Clapham, D. E. (1993). Specificity of receptor-G protein interactions Searching for the structure behind the signal. Cell. Signal. 5, 505-518. 

Heydorn, A., Ward, R. J., Jorgensen, R., Rosenkilde, M. M., Frimurer, T. M., Milligan, G., and Kostenis, E. (2004). Identification of a novel site within G protein a subunits important for specificity of receptor-G protein interaction. Mol. Pharmacol. 66, 250-259. 

Receptor-G protein interactions have been monitored with optical techniques both via FRET and BRET (Gales et at, 2005 Hein et at, 2005). Even though there are important differences in details of the results obtained, the general conclusions from the two approaches are similar. The interaction between activated receptors and their G proteins can be very rapid and can, at high expression levels, reach the speed of receptor activation itself (i.e., occur with a x in the 30- to 50-ms range, Hein et al, 2005). 

On the level of kinetics it is interesting to note that activation of G proteins by receptors appears to be rather slow. For example, in the case of -adrenergic receptors and G , the comparison of receptor—G protein interaction kinetics, which occur with time constants of 30—50 ms (Hein et al, 2005), with the kinetics of G activation, which require 0.5-1 s (Biinemann et al, 2003), reveals that the G proteins may be a time-limiting step in this signaling pathway. It remains to be seen whether the release of GDP from the a subunit, which is induced by the agonist-occupied receptor, is indeed the reason for the slow kinetics of G protein activation, as may be inferred from biochemical experiments in cell membranes and in reconstituted systems. 

Later, in order to account for the effects of a point mutation on the activity of the p2-adrenergic receptor, Samama et al. [29] have proposed an extended version of the ternary complex model. In this model the receptor molecule exists in an equilibrium between the inactive R and the active R conformations. In the absence of ligand, the ability of the receptor to spontaneously convert from the inactive to the active conformation is determined by the isomerization constant, J. The active R conformation is the molecular species that enters into productive interaction with the G protein, described by the equilibrium constant M. The values of both J and M are dependent only on the receptor-G protein system, and are independent of the presence or absence of ligand. The ability of different ligands to perturb this equilibrium is gauged by the ligand-specific equilibrium constant (5, the... 

G-protein linked receptors, sometimes referred to as seven membrane spanning serpentine receptors include, as their name suggests, seven membrane-spanning sequences. Sites of receptor/G-protein interaction are within cytoplasmic and possibly membrane spanning sequences. Recent evidence indicates that die carboiqrl terminal portion of the third intracellular loop and the carbojqrl terminal portion of the receptor are the likely sites of interaction.(Bimbaumer, Bimbaumer, 1995). 

Ulfers AL, McMurry JL, Miller A, Wang L, Kendall DA, Mierke DF (2002b) Cannabinoid receptor-G protein interactions G(alphail )-bound structures of IC3 and a mutant with altered G protein specificity. Protein Sci 11 2526-2531... 

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