6F: Signaling through G-Protein Coupled Receptors

Question 1

GPCRs

Answer 1

• 7 transmembrane domains
• many drugs target GPCRs
• receptor for many ligands
• binds and activates heterotrimeric G proteins

Question 2

subunits of heterotrimeric G proteins

Answer 2

alpha, beta, gamma, and the bound GDP

Question 3

How does GPCR signaling occur?

Answer 3

• the ligand binds and causes a conformational change in the receptor
• this change in the receptor causes a conformational change in the alpha subunit of the G protein
• this change in the alpha subunit causes the GDP to be exchanged for GTP
• alpha+GTP and the beta+gamma subunits dissociate so that they can interact with effector molecules
• binding of the effectors trigger downstream signaling events (cellular response or gene transcription)

Question 4

Why don’t the inactive G protein subunits bind to their effectors randomly?

Answer 4

because when they are inactive and all of the subunits are bound together, the critical binding portions are closed off

Question 5

How do GCPRs have specificity?

Answer 5

• There are many different types of alpha, beta, and gamma subunits that can be used to create the G-protein
• also, different receptors have different Kd’s for different ligands

Question 6

How are G-proteins turned off?

Answer 6

• The alpha subunit is also an enzyme with intrinsic GTPase activity
• RGS proteins (regulator of G proteins) bind to the activated alpha subunit and accelerate the rate at which that GTP can be hydrolyzed
• this causes re-association of the G-protein subunits
• RGS proteins are a type of GAP
  But this isn’t enough, because even though we can inactivate the protein, the receptor is still active and the whole process will happen again

Question 7

Heterologous desensitization (of the GCPRs)

Answer 7

• works on a broad classes of GCPRs
• PKAs and PKCs are activated downstream of GCPR signaling
• they then phosphorylate the receptors - ligand bound (activated) or not ligand bound (inactivated)
• this phosphorylation introduces a barrier to interacting with the G-protein

Question 8

Homologous desensitization (of the GCPRs)

Answer 8

• only on ligand bound GCPRs
• triggered by recruitment of GRKs to the membrane
• GRK then binds to the beta+gamma activated subunit
• GRK then phosphorylates the GCPR
• Now the phosphorylated GCPR will bind ARR
• ARR then block GCPR binding with the alpha subunit
• Arrestin (ARR) also promote internalization and either dephosphorylated or degradation of the GCPR

Question 9

GCRP affinity for ligands

Answer 9

GCPRs bound to the heterotrimer G-protein have a much higher affinity for ligand than GCPRs not bound to G-proteins

Question 10

relation of G-proteins and cholera

Answer 10

in cholera, the cholera toxin ADP ribosylates the active alpha subunit. Because of this there is loss of the GTPase activity of the alpha subunit. Sooo the alpha subunit is always active (no ligand present), which leads to up-regulation of a downstream event and massive diarrhea

Question 11

relation of G-proteins and pertussis

Answer 11

The pertussis toxin ADP ribosylates the inactive alpha subunit, so this makes it so that the G-protein can’t bind to the ligand bound receptor. This means no G-protein coupled responses

Question 12

relation of G-proteins and Jansen’s

Answer 12

mutation in the PTH receptor. Constitutive activate of the GCPR, which leads to ligand-independent G-Protein activation

Question 13

How can GCPRs lead to overdose in drugs?

Answer 13

1. chronic use can desensitize the receptor, which shifts it’s curve to the right. So much so that in order to get the desired effect, they have to use what’s considered a “lethal dose” that triggers other cascades and leads to death
2. chronic use can shift the curve to the right and then the user quits cold turkey. They then take the dose that they normally would have with the desensitized receptor, and this is now a lethal dose for their resensitized receptors