Gai-GTP

Adenylyl cyclase catalyses the reaction ATP— cAMP + pyrophosphate (PP,). Membrane-bound ACs are activated by hormones and NTs that act via G protein-linked receptors to generate AC-activating Gas GTP (Chapter 5). Particular AC isoforms are activated by Ca2+-calmodulin, this representing an example of cross-talk between cAMP and Ca2+ signalling pathways. As outlined in Chapter 5, Gai GTP inhibits AC and hence lowers cAMP concentration. A variety of hormones and NTs act via GPCRs to either activate or inhibit AC and in turn a variety of plant-derived compounds interfere with these processes (Chapter 5). The plant-derived diterpene forskolin and related compounds directly activate AC (Table 7.2). 

In contrast, pertussis toxin catalyzes the ADP-ribosyl-ation of a specific cysteine residue in Gai) G(m and Gal [1]. Only a subunits bound to their Py subunits can undergo this modification. Pertussis-toxin-mediated ADP-ribosylation inactivates these a subunits such that they cannot exchange GTP for GDP in response to receptor activation (Fig. 19-1B). By this mechanism, pertussis toxin blocks the ability of neurotransmitters to inhibit adenylyl cyclase or to influence the gating of K+ and Ca2+ channels in target neurons. However, since G is not a substrate for pertussis toxin, the toxin may not be able to block neurotransmitter-mediated inhibition of adenylyl cyclase in all cases. The Gq and Gn 16 types of G protein a subunit are not known to undergo ADP-ribosylation. 

Agonist occupancy of GPCRs, such as the delta opioid receptor, leads to physiological effects through interactions with heterotrimeric G proteins. Such G proteins consist of a Ga subunit and its Gpy dimeric partner. There are four major families of Ga proteins with different profiles of effector interaction 1) Gas, which activate adenylyl cyclase 2) Gai/o, so-called inhibitory G proteins named for their ability to inhibit adenylyl cyclase, but interact with many effectors 3) Gaq/11, which activate phospholipase C- 3 (PLC- 3) and 4) Gal2/13, which may regulate small GTP-binding proteins. Delta opioid receptors, like mu and kappa opioid receptors, couple to mem-... 

Got subunits of the Gi/o type of G proteins can be ADP-ribosylated in the presence of pertussis toxin at Cys351, four amino acids from the C-terminus. Petussis toxin sensitivity is the major method of identifying a role for Gai/o proteins in GPCR-mediated signaling. This treatment prevents receptor-mediated G-protein activation and thus exchange of GTP for GDP and so blocks signaling by Ga and G(3y. There are numerous examples of the use of this technique to identify coupling of the delta opioid receptor [e.g., 2,3,41,42,73,77]. One Ga protein in this class, Gaz, lacks the Cys residue that is the site for pertussis toxin action and so is insensitive to pertussis toxin treatment [see 17 for review]. 

All three heterotrimeric G proteins are required for GPCR coupling [51, 52], Moreover once GTP binds both the Ga- and Gpy-subunits can activate effector proteins and ion channels, such as AC, phospholipases C, Ca2+ and K+ channels [81]. For example, while the activated Gas tends to activate AC [82, 83], Gai tends to inhibit AC, and activated Gaq tends to activate phospholipase C-p [39, 84], Variations in receptor structure can change the rate at which G protein subunits are liberated. Enhanced or diminished GPCR signaling can result from changes in these processes at any step. 

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