G Protein Mediated Signalling I

Question 1

CaM-kinase II ‘memory switch’

Answer 1

Ca2+/calmodulin = CaM-kinase II active
- auto-phosphorylation (more kinase activity)
- Ca2+ and calmodulin leave = still active
- remove phosphate to inactivate

Question 2

Deactivation of G Proteins

Answer 2

RGS proteins – deactivate
Phosducins – limit Gβγ formation
GDI – block Gβγ bind to Gα
Kinase – phosphorylate GPCR (less signal to G proteins)
Arrestin – impede Gα binding

Question 3

Reciprocal Regulation

Answer 3

PLCβ is a GAP for Gαq
– Gαq = PLCβ hydrolyze PIP2
– PLCβ = Gαq hydrolyze GTP

Question 4

RGS Proteins Function

Answer 4

• turn off G-proteins (opposite GPCR)
• GAPs = bind to G-proteins, increase rate GTP hydrolyze
• limit G protein mediated signalling (Gi/o and Gq)

Question 5

RGS Protein Subfamilies

Answer 5

R4 and RZ = RGS domain + short N/C termini
R7 = GGL (bind to Gβ5) + DEP (bind site anchor proteins)
R12 = GDI-dissociation inhibitor (no activate/GTP bind)

Question 6

Physiological Roles of RGS Proteins

Answer 6

Desensitization – decrease receptor response
Maintain Rapid Signal Kinetics – turn off G-protein fast
Muting/Dimming – reduce spontaneous G activation
Selective Effects – targeted

Question 7

GPCR Desensitization

Answer 7

• loss of receptor response with continued stimulation

1) phosphorylation (lower agonist ability activate G)
2) internalization (remove receptor from surface)
3) down-regulation (decrease receptor number)
— increase degrade OR decrease mRNA level

Question 8

GPCR Desensitization Phosphorylation

Answer 8

Decrease receptor responsiveness to agonist
- cAMP –> PKA and DAG/Ca2+ –> PKC
Arrestin
- GRK = kinase, phosphorylate activated GPCR
- arrestin binds and compete with G-protein for receptor

Question 9

GPCR Desensitization Endocytosis

Answer 9

• β-arrestin target GPCR to clathrin-coated pit
• internalize receptor (no bind agonist)

Question 10

β-arrestin Associations

Answer 10

Rapid Dissociators (β2-adrenergic receptor)
- rapid dissociate from β-arrestin
- receptor recycled to PM rapidly

Slow Dissociators (angiotensin receptor)
- form stable complexes with β-arrestin
- receptors recycle to PM slowly

Question 11

Arrestin and RGS9 in Light Adaptation

Answer 11

Dark = G-protein bound to receptor
Light = Arrestin translocate to receptor