M&R 7.1 - Receptor-effector signalling via G-proteins Flashcards
What does ‘G-protein’ stand for?
Guanine nucelotide binding protein
Describe the common basic structure of a GPCR
Single polypeptide chain with 7TM domains
Extracellular N-terminal, intracellular C-terminal
Binding site can be formed by 2-3 of the TM domains, or by the N terminal region
Generally, ligand binding to a GPCR causes….
… a conformational change which allows the receptor to bind the G-protein
Describe the structure of a G-protein
Heterotrimers - have 3 distinct subunits (alpha, beta & gamma)
beta & gamma subunits bind tightly to each other and function as a single unit
Can G-proteins move around in the plasma membrane?
Yes - this allows them to approach the activated GPCR
What intrinsic activity does the alpha subunit of a G-protein have?
GTPase activity - it binds GTP and slowly hydrolyses it to GDP
Under basal conditions, what form is the G protein in? What is bound to the alpha subunit?
G-protein is in its heterotrimeric form
The alpha subunit has GDP bound to it
Why does the G-protein bind to the receptor? What happens?
Once activated, the receptor has high affinity for the basal form (heterotrimer with GDP bound) of the G-protein
Binding results in the alpha subunit of the G-protein giving up its GDP, and GTP binds in its place
What happens once GTP is bound to the alpha subunit of the G-protein?
The alpha-GTP subunit of the G-protein loses its affinity for the receptor and for the beta-gamma subunit
Therefore the alpha-GTP subunit is released, and the beta-gamma subunit is released
Both can go off and interact with effectors (e.g. enzymes, ion channels)
How is G-protein signalling terminated?
By the intrinsic GTPase activity of the alpha subunit - gradually hydrolyses its GTP back to GDP
Once back to alpha-GDP, it rediscovers its high affinity for the beta-gamma subunit
Therefore the inactive G-protein heterotrimer is reformed
What is the primary determinant for the response of a GPCR?
The variant of alpha subunit on the G-protein associated with it
Which G proteins alpha subunits are associated with which adrenoceptors?
Alpha1 - q
Alpha2 - i
Beta1 - s
Beta2 -s
Which G-protein alpha subunits are associated with which muscarinic receptors?
M1 - q
M2 - i
M3 - q
(M4 - i)
Which G-protein alpha subunit is associated with rhodposin ( a light sensitive GPCR)
t (=transducin - activates cGMP phosphodiesterase)
How does cholera toxin affect the activity of G-proteins?
Inhibits GTPase activity of G-proteins with a Gs-type alpha subunit
This causes constant activation of Gs-mediated signalling pathways (because GTP not hydrolysed to GDP to terminate the signal)
