G proteins activation

The role of ATP in muscular contraction has parallels to the role of GTP in G-protein activation... 

As described in Section 3.13.6, the fraction p of G-protein-activating species (producing response)—... 

The most probable mechanism for inverse agonism is the same one operable for positive agonism namely, selective receptor state affinity. However, unlike agonists that have a selectively higher affinity for the receptor active state (to induce G-protein activation and subsequent physiological response) inverse agonists have a selectively higher affinity for the inactive receptor state and thus uncouple already spontaneously coupled [RaG] species in the system. 

It is assumed that the receptor species leading to G-protein activation (and therefore physiological response) are complexes between the activated receptor ([RJ) and the G-protein namely, [ARaG] + [RaG], The fraction of the response-producing species of the total receptor species (([ARaG] + [RaG])/Rtot) is denoted p and is given by... 

Finally, receptor stimulus can be measured through membrane assays directly monitoring G-protein activation (group IV assays). In these assays, radiolabeled GTP (in a stable form for example, GTPj/S) is present in the medium. As receptor activation takes place, the GDP previously bound to the inactive state of the G-protein is released and the radiolabeled GTP/S binds to the G-protein. This is quantified to yield a measure of the rate of GDP /GTP j/S exchange and hence receptor stimulus. 

Abd Alla, S., Lother, H., and Quitterer, U. (2000). Atl-receptor heterodimers show enhanced G-protein activation and altered receptor sequestration. Nature 407 94-98. 

FIGURE 11.3 One-way ANOVA (analysis of variance). One-way analysis of variance of basal rates of metabolism in melanophores (as measured by spontaneous dispersion of pigment due to G,.-protein activation) for four experiments. Cells were transiently transfected with cDNA for human calcitonin receptor (8 j-ig/ml) on four separate occasions to induce constitutive receptor activity. The means of the four basal readings for the cells for each experiment (see Table 11.4) are shown in the histogram (with standard errors). The one-way analysis of variance is used to determine whether there is a significant effect of test occasion (any one of the four experiments is different with respect to level of constitutive activity). 

There is abundant experimental evidence suggesting that many (if not most) GPCRs exist as homodimers and/or heterodimers [4]. The functional significance of dimers and their role in G protein activation remains to be determined for most GPCRs. Experimental evidence suggests that dimers may play roles in processing and maturation of newly synthesized receptor protein. Heterodimers may contribute to pharmacologic... 

Kir3.2 Weaver mouse. A mutant mouse with cer ebellar degeneration and motor dysfunction resulting from a serine for glycine substitution in the -GYG- sequence of the K selectivity filter of Kir3.2. G-protein activated K conductances are abolished in the cerebellar neurons, leading to Ca2+ overload and cell death. 

The 3 isozymes are activated by G protein-coupled receptors through two different mechanisms [2]. The first involves activated a-subunits of the Gq family of heterotrimeric G proteins (Gq, Gn, Gi4, G15/16). These subunits activate the (31, (33 and (34 PLC isozymes through direct interaction with a sequence in the C terminus. The domain on the Gqa-subunit that interacts with the (3 isozymes is located on a surface a-helix that is adjacent to the Switch III region, which undergoes a marked conformational change during activation. The second mechanism of G protein activation of PLC 3 isozymes involves (3y-subunits released from Gi/0 G proteins by their pertussis toxin-sensitive activation by certain receptors. The 3y-subunits activate the 32 and 33 PLC isozymes by interacting with a sequence between the conserved X and Y domains. 

All rhodopsin-like G-protein-coupled receptors have a conserved arginine residue at the intracellular end of TM3 and this residue is thought to be crucial for G-protein activation. The third intracellular loop determines the class of G-protein activated by the receptor with the second intracellular loop and C-terminus also influencing G-protein binding in some cases. Four classes of G-protein are known ... 

Haskell CA, Cleary MD, Charo IF. Molecular uncoupling of fractalkine-mediated cell adhesion and signal transduction. Rapid flow arrest of CX3CR1-expressing cells is independent of G-protein activation. J Biol Chem 1999 274(15) 10053-10058. 

G-protein activation has a cyclical nature. The a subunit can hydrolyze the GTP that is bound to it, thereby allowing the heterotrimer to reform. The lifetime of individual aGTP subunits will vary (cf. the lifetimes of open ion channels). 

This term has increasingly come to be employed (as here) in a rather different sense from that introduced by R. F. Furchgott (Section 1.4.2). With this newer usage, intrinsic efficacy indicates the maximum receptor activation (often expressed as the fraction of receptors in the active state) that can be achieved by an agonist acting through a mechanism that can be formulated and studied at the molecular level, as in the present example. The intention of this redefinition is to focus on the receptor itself and its immediate transduction mechanism (e.g., G-protein activation), rather than on the cellular events that follow. 

Kd 50 nM for GIRK [G-protein-activated inwardly rectifying K+ channel] activation) and persistent in the absence of competing Ga-GDP, Py-effector binding is not irreversible. 

Abbreviations 3ARK1, -adrenergic receptor kinase 1 PLC, phospholipase C GIRK, G-protein-activated inwardly rectifying potassium channel Ca(N, P, Q), N-type, P-type, or Q-type calcium channel. 

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