Guanyl nucleotide-binding protein

Adenylate cyclase is a two-component enzyme system. It ultimately catalyzes the cyclase reaction, but only when it is associated with the hormone-bound receptor and a regulatory protein called a stimulatory G-protein (guanylate nucleotide binding protein), which activates adenylate cyclase. The G-protein is the intermediate between the receptor and the synthesis of cyclic AMP. 

G alpha(q) (Guanyl Nucleotide Binding Protein G Alpha q)... 

G protein, heterotrimeric guanyl nucleotide-binding protein... 

Vanadate(V) and GTP-y-S, a hydrolysis resistant analog of GTP, have recently been shown to excite photoreceptors of the horseshoe crab Limulus in the dark and have been used to help elucidate the chemical steps which are involved in the phototransduction process153. The results suggest that vanadate binds to a guanyl nucleotide binding protein which activates adenylate cyclase. 

D. Cassel and T. Pfeuffer. Mechanism of choleratoxin action Covalent modification of the guanyl nucleotide-binding protein of the adenylate cyclase system. Proc Natl Acad Sci, USA, 75, 2669, 1978. 

Locate guanyl nucleotide-binding proteins (G proteins) in the cell and describe their structures, catalytic characteristics, and molecular mechanisms of activation and inactivation. Describe the roles of G proteins in coupling a hormone-receptor complex to adenylate cyclase and in amplifying the stimulus. 

Guanyl nucleotide-binding proteins (G proteins) have vhich of the following properties ... 

Microtubules have a key role in mitosis and cell-proliferation. They are dynamic assemblies of heterodimers of a and f3 tubullin. In the cell-reproduction cascade tubulin polymerizes fast and subsequently depolymerizes. Tubulin dimers are unusual guanyl nucleotide binding (G) proteins, which bind GTP reversibly at a site in the (3-tubulin. GTP irreversibly hydrolyzes to GDP during polymerization. 

Ebinn, J.O., Bottorf, D.A., Chan, E.Y.W., Stang, S.L., Dunn, R.J. and Stone, J.C. RasGRP, a ras guanyl nucleotide-releasing protein with calcium- and diacylglycerol-binding motifs (1998) Science 280,1082-1086... 

Moss J, Vaughan M (1988b) ADP-ribosylation of guanyl nucleotide-binding regulatory proteins by bacterial toxins. In Adv. Enzym. 61 303-379. 

The receptors (R) interact with a complex (GE) of the adenylate cyclase catalytic subunit (E) and the guanyl-nucleotide binding regulatory protein (G) with a 1 1 stoichiometry and (t/) complexes of GE are activated when an equal number (y) of receptors are occupied. 

Figure 8.27 Arrangement of the constituent proteins in the lipid bilayer of a human platelet. The diagram shows a schematic representation of the receptors (R) for PGI2 and adenosine (AJ, the catalytic subunit of adenylate cyclase (E), and the 3 subunits (a, jS and y) of the guanyl nucleotide-binding regulatory protein (GJ...

Initial biochemical studies indicate that agonist binding was regulated by guanyl nucleotides, implying that the receptor belongs to the superfamily of receptors coupled to G proteins. In addition, various intracellular responses were found to be associated with Hi-receptor stimulation inositol phosphate release, increase in Ca2+ fluxes, cyclic AMP or cyclic GMP accumulation in whole cells and arachidonic acid release [1],... 

G-proteins exist either in an active or an inactive state, depending on the guanylate nucleotide that is bound. In the inactive state, G-protein binds to GDP. In the active state, GTP is bound to the G-pro-tein. G-proteins have an intrinsic GTPase activity, which converts bound GTP to GDP. Hydrolysis of GTP by the G-protein converts the G-protein back to an inactive state. Thus the cycle of the G-protein is as follows ... 

How do these G proteins operate In the unactivated state, the guanyl nucleotide bound to the G protein is GDP. In this form, the G protein exists as a heterotrimer consisting of a, P, and y subunits the a subunit (referred to as G ) binds... 

In the G protein s unactivated state, the guanyl nucleotide bound to the G protein is GDP. In this form, the G protein exists as a heterotrimer consisting of a, p, and y subunits the ct subunit (referred to as G ) binds the nucleotide (Figure 14.6). The a subunit is a member of the P-loop NTPase family (Section 9.4), and the P-loop participates in nucleotide binding. The a and y subunits are usually anchored to the membrane by covalently attached fatty acids. The role of the hormone-hound receptor is to catalyze the exchange of GTP for hound GDP. The hormone—receptor complex interacts with the heterotrimeric G protein and opens the nucleotide-hinding site... 

G-protein receptors are so named because their action is linked to the binding of guanyl nucleotides. 

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