Plasma membrane receptors which couple to heterotrimeric G proteins. Flashcards
What does GPCR stand for?
G-protein coupled receptor.
Are GPCR proteins a type of metabotropic receptor?
Yes. But they often indirectly cause channels to open.
What type of G-protein do GPCR receptors entail?
- Monomeric?
- heterotrimeric
heterotrimeric:
alpha, beta, gamma.
How many genes in percentage and number deal with GPCRs?
~800 genes
~4% of all genes
and 40-50% of clinical drugs affect this region
In a GPCR what does the Galpha subunit do?
It functions as the actual molecular switch. It possesses intrinsic GTPase activity
Which of the three subunits of a heterotrimeric g protein are imbedded in the membrane?
Through what means?
alpha and gamma
Prenylation
Where in the alpha subunit does the GTP/GDP reside?
In the AH domain!
How does the beta subunit stay localized with the other subunits? (since it is not connected to the membrane)
It dimerizes with the gamma subunit.
What is true when the Galpha subunit is bound to GDP?
It is mostly inactive.
It has a high affinity for beta-gamma subunit.
What is the difference between a GPCR and a heterotrimeric G protein?
A GPCR is a receptor. It is coupled and interacts with a heterotrimeric G-protein.
How does the GPCR interact with the heterotrimeric G protein?
If the GPCR is ligand activated or constitutively active, then it will act as a GEF. It will remove GDP, allowing GTP to bind and more importantly separation of the Galpha subunit and the G-beta-gamma subunits. Exposing the interaction surfaces.
What is the concentration of GTP relative to GDP in the cytosol?
GTP ~.5mM
GDP ~.05mM
How is it possible that in some cells the Galpha and Gbetagamma of activated G-proteins do not dissociate?
While they do not disassociate, binding of GTP results in a shift which exposes the interactive surfaces of alpha and beta.
What does adenylyl cyclase do? It is activated or inhibited by which G subunit?
It turns AMP to cAMP.
It is activated by Galpha(s) subunit
How are VGSCs affected by the Gbetagamma subunit?
Inhibited.
Effector antagonist:
Downstream effector blocks signaling of G-protein subunit. Negative feedback. (do NOT use a GAP mechanism)
GRKs (G protein-coupled receptor kinases) do what?
They phosphorylate GPCRs.
How many types of GRKs are there?
At least three
GRK1
GRK2
GRK3
Name to protein kinases which are not GRKs which phosphorylate GPCRs.
Protein Kinase A (PKA)
Protein Kinase C (PKC)
Only _______ GPCRs are phosphorylated by GRKs.
activated.
What are the steps of arrestin mediated internalization?
An activated GPCR is phosphorylated by a GRK (or PKA or PKC).
After phosphorylation, arrestin can bind, inhibiting GPCR from interacting with G-proteins.
Arrestin is an adaptor for clathrin, which causes endocytosis.
internalized GPCRs can be destroyed in a lysosome, or signal in a different what after arrestin recruits intracellular signaling proteins.
What are the four mechanisms to block or reduce signaling through GPCRs?
Inactivation: involves covalent modification of GPCR proteins so they lose GEF activity.
Sequestration: GPCRs are endocytosed into recycling endosomes, and are not exposed to their agonist, can be sent back the the PM.
Desensitization: same mechanism as inactivation.
Down-regulation: GPCRs are endocytosed and destroyed within lysosomes.
Why are yeast used as a model to study GPCRs?
GPCR & and heterotrimeric g-proteins are evolutionarily conserved mechanisms between yeast and humans.
Yeast mating factor is similar to what human hormone?
What does it do?
It is similar to LH (produced by the anterior pituitary)
Causes a behavior in haploid yeast called shmooing, where they extend in the direction of yeast mating factor.
What type of receptor system deal with yeast mating factor?
GPCR agonist.
How long does the intrinsic hydrolytic ability of alpha subunit take to GTP–>GDP?
~10 minutes
What do (RGS) Regulators of G protein signaling do?
They act as GAPs. by catalyzing hydrolysis GTP –> GDP in Galpha subunits. Once the Galpha has GDP bound, it regains affinity for a Gbetagamma subunit.
How many distinct RGS proteins exist? How do they tie in with visual phototransduction?
~30
allow for rapid down regulation of visual signal so another visual signal can be perceived.
How much do RGS accelerate GTP hydrolysis?
Do they affect the onset and termination?
1000x
yes
Effector gap:
Form of negative feedback where a downstream effector will act as gaps for the Galpha subunit (PLC-beta1 does this for gap Galpha(q)/11)
Name the Galpha subunits downstream effector: Galpha(s) Galpha(i/o) Galpha(g/11) Galpha(12/13)
Galpha(s): stimulates adenylyl cyclase
Galpha(i/o): inhibits adenylyl cyclase
Galpha(q/11): stimulates phospholipase C
Galpha(12/13): activates rhoGEF
What does rhoGEF do?
It is the GEP for Rho/rac/cdc42 family, which regulation cytoskeleton and therefor shape and locomotion.
Cholera toxin does what mechanistically?
ADP-ribosylation of Galpha(s), this blocks GTP hydrolysis, keeping constitutive activity. Causes diarrhea, cholera.
Pertussis toxin does what mechanistically?
PTX causes Galpha(i) to by ADP-ribosylated, blocks activation of G(i/o) by GPCR. causes whooping cough.
what do they do? Galpha(s): Galpha(olf): Galpha(i/o): Gbetagama: Gbetagama: Galpha(T): Galpha(q/11):
Galpha(s): stimulates adenylyl cyclase (AC)
Galpha(olf): activates olfactory adenylyl cyclase (AC)
Galpha(i/o): inhibits AC
Gbetagama: activate GIRK potassium channels.
Gbetagama: inhibits VGCaCs.
Galpha(T): direct activation of phosphodiesterase in rods and cones
Galpha(q/11): directly activates phospholipase beta1
