16. GCPRs and Signal Transduction

Question 1

Why are GPCRs important in the body?

Answer 1

Signal transduction by GPCRs is funcdamental or most physiological processes.
* vision, smell, taste: neurological, reproductive functions, etc

Question 2

How many transmembrane spanning regions do GPCRs have?

Answer 2

7 regions

Question 3

Where is the N-terminus located versus the C-terminus?

Answer 3

N-terminus = out
C-terminus = in the cytoplasm

Question 4

What are some structural characteristics of GCPRs?

Answer 4

• anchored at the C-terminus
• 3 intracellular loops
• 3 extracellular loops

Question 5

What modification usually occurs on the N-terminus? C-terminus?

Answer 5

N-terminus –> glycosylated
C-terminus –> palmitoylated
* palmitoylated groups anchor the C-terminus into the membrane

Question 6

What are G proteins?

Answer 6

3 proteins that associate with with the GPCRs inside the cytoplasm
* alpha, beta, and gamma

Question 7

What do alpha g-proteins bind?

Answer 7

Alpha g-proteins bind GTP

Question 8

Explain the process of GPCR activation by ligand binding.

Answer 8

• When the ligand binds, GDP leaves and the alpha subunit becomes occupied by GTP.
• alpha subunit goes on to initiate signaling
• beta and gamma subunits stay together to form heterodimer and go down their own signaling pathway

Question 9

What is the function of RGS?

Answer 9

Stimulate hydrolysis of the phosphate bond
* GTP –> GDP
* returns G-alpha to inactive state

Question 10

How do G-proteins get activated?

Answer 10

1. ligand binds
2. conformational changes in the GCPR
3. G-protein activation

Question 11

How many ligands can one GPCR bind to?

Answer 11

Usually 1 GPCR can bind to 1 ligand

Question 12

Describe the cycle of GPCR activity.

Answer 12

1. resting (GDP-bound) state
2. ligand binds –> GDP exchanged for GTP and G-alpha dissociates
3. G-alpha associates with an effector (G-alpha can diffuse through the membrane)
4. Effectors generate second messengers
5. GTP gets hydrolyzed to GDP by RGS proteins –> inactive

Question 13

What are the four families of G-alpha proteins?

Answer 13

• Gi - inhibits adenlyate cyclase; open K+; close Ca2+
• Gs - activates adenylate cyclase
• Gq - activate phospholipase C
• G12/13 - activate Rho GTPases

Question 14

Explain the process of adenylate cyclase coupled receptor activation. (secondary messenger)

Answer 14

1. ligand binds D1 or D5
2. GDP leaves and GTP associates with G-proteins
3. Gs activates adenylate cyclase
4. ATP –> cAMP
5. cAMP activates protein kinases
6. Kinases can trigger responses in the cytoplasm OR phosphorylate transcription factors
7. Transcription factors can be transported into the nucleus and activate gene expression

Question 15

Cyclic AMP second messenger systems: What is a reoccurring characteristic of the messenger system?

Answer 15

The kinases in the messenger pathway must be activated
* activation/activity is caused by phosphorylation by another kinase

Question 16

Describe the phosphoinsotide pathway.

Answer 16

1. Gq activates phospholipase C
2. Splits phospholipase C into two: diacylglycerol and IP3
   * Diacylglycerol activates protein kinase C
   * IP3 activates Ca2+ channels in the endoplasmic reticulum –> activates protein kinase C

Question 17

What can G-proteins affect?

Answer 17

Channel activity and enzymatic function

Question 18

Explain the functional difference between GEF and GAP.

Answer 18

GEF –> positively regulates proteins (replaces GDP with GTP)
GAP –> negatively regulates proteins (hydrolyzes phosphate - GTP –> GDP)