GPCR βγ Regulation Flashcards
What was the initial model of GPCR signalling predicated upon? and what was Gβγ subunit’s role in this model?
GTP-bound Gα subunit conveying the transduced signal to the effector
Gβγ subunit’s only role was to obstruct the Gα effector-binding domains in the absence of ligand or act as an anchor to the plasma membrane
What and who challenged the initial model of GPCR signalling?
Critical finding of David Clapham’s group (1987) that purified Gβγ subunits from the brain, and not Gα subunits, were responsible for transducing the signal of muscarinic acetylcholine receptors to activate GIRK channels in cardiac pacemaker and atrial cells
Who and how were initial findings of Gβγ activity disputed?
Groups of Birnbaumer (1987) and Brown (1988)
evidence that the activating component of the heterotrimer, then termed “Gk” after the potassium channel, was a G subunit and that the purified Gβγ preparation of Clapham’s group may have been slightly contaminated with a high-affinity Gα, or interference was generated somehow from CHAPS detergent
Which lab found similar results to Clapham?
Kurachi, 1989
How was the importance of Gβγ finally demonstrated?
Jan (1994) and Lester (1995), facilitated by the development of heterologous expression systems such as Xenopus oocytes, and the cloning of the individual cDNAs responsible for the G proteins αi, β and γ which implicated Gβγ as the signal transducer of GIRK
After GIRK’s, what other modulation was found through technological advances in G protein isolation?
Catterall (1996) determined that the active G protein subunit responsible for Gi/o-coupled GPCRs’ inhibitory action upon N and P/Q-type voltage-gated calcium channels was Gβγ and not Gα
This finding was echoed by Ikeda (1996) for N-type channels in the same issue of Nature.
Who and what other pathways were shown to be activated by Gβγ?
Harden (1991) and Gilman (1992) that phospholipase C and type II adenylyl cyclase (Tang et al., 1991) were activated by Gβγ provided more widespread acceptance that Gβγ– effector interactions were a critical aspect of G protein signalling.
Gβγ coupled to which α subunits effects GIRKs - and how?
Gβγ dimers released from pertussis toxin (PTX)-sensitive (Gαi and Gαo) G proteins bind directly to GIRK channels, causing them to open
What are GIRK’s?
GIRK channels are members of a large family of inwardly rectifying K+ channels (Kir1–Kir7)
What is inward rectification?
A change in slope of the current–voltage relationship of the channel at the reversal potential (that is, the zero-current level, which occurs at the equilibrium potential for K+ (EK)). At membrane potentials well above EK, the outward current is very small compared to the inward current at potentials well below EK
Why does rectification occur?
This rectification is due to occlusion of the central pore by intracellular Mg2+ and polyamines at potentials above EK
What function do GIRK’s typically serve?
Under physiological conditions, the resting membrane potential of a typical neuron is positive to EK, and the small outward K+ current through GIRK channels decreases the excitability of a neuron.
Examples of neurotransmitters that act on GIRKs
acetylcholine, dopamine, opioids, serotonin, somatostatin, adenosine and GABA , activate these channels by stimulating their cognate GPCR’s, which couple specifically to PTX-sensitive heterotrimeric G proteins that activate GIRK channels
Where does Gβγ bind on GIRK and how do we know this?
Studies using chimeras and biochemistry demonstrated that Gβγ subunits bind directly to the N-terminal domain and the distal part of the C-terminal domain of GIRK1 (Huang et al. 1995; Inanobe et al. 1995; Kunkel & Peralta, 1995; Slesingeret al. 1995).
How do we know the C-terminal domain is particularly important for Gβγ binding to GIRK?
Krapivinsky et al. (1998) examined the effects of synthetic peptides derived from GIRK1, GIRK4 or IRK1 on the biochemical binding of Gβγ to GIRK1/4 channels and concluded that two regions within the C-terminal domain were important for Gβγ binding and activation.
