G Protein Coupled Receptor II

Question 1

Gα Subfamilies

Answer 1

Gαs – activates adenylyl cyclase
Gαi/o – inhibits adenylyl cyclase
Gαq – activates PLCβ + p63RhoGEF
Gα12/13 – activates multiple RhoGEF

Question 2

Gβ and Gγ Genes

Answer 2

Gβ1-Gβ4 = similar (Gβ5 does not interact with Gγ)
12 Gγ genes – forms stable dimers with Gβ
— regulates adenylyl cyclase, PLCβ, voltage-gated Ca2+ channels

Question 3

Coupling to multiple G-proteins

Answer 3

One receptor –> activate multiple G-proteins
– LPA1 = Gi + G12/13 + Gq

Multiple related receptors activated by same agonist can activate different G-proteins
– D1 and D2 dopamine receptors = Gs + Gi

Question 4

Adenylyl Cyclase Structure and Regulation

Answer 4

• alternate hydrophobic (6 TMD) and hydrophilic domains (catalytic function – make cAMP)

Gαs = all 9 isoforms activate
Gαi = some isoforms inhibit
Gβγ = some isoforms inhibit, some activate
Ca2+ = less activity
Calmodulin = more activity
PKA = less activity (negative feedback)
PKC = less/more activity

Question 5

Structure of PKA

Answer 5

• serine/theronine kinase
• 2 catalytic subunit & regulatory subunit dimer (inactive)
  Type 1 = cytoplasmic
  Type 2 = cellular structures (localized by regulatory subunit and A kinase anchoring proteins)

Question 6

cAMP and PKA Pathway

Answer 6

activated adenylyl cyclase = more cAMP
- 2 cAMP binding sites per regulatory unit
- dissociate catalytic subunits (active + phosphorylate)

PKA phosphorylates
- receptors, ion channels
- CREB (binds to CRE– regulate gene transcription)

Question 7

Other cAMP-activated Pathways

Answer 7

Cyclic nucleotide-gated ion channels (pacemaker of HR)
EPAC
- cAMP activates EPAC activates Rap1 –> exocytosis

Question 8

Cyclic Nucleotide Phosphodiesterase

Answer 8

Selective
- cAMP = PDE2a, PDE3/4/7/8 || cGMP = PDE1a, PDE1b

Regulatory Mechanisms
- calmodulin activates PDE1

Kinase Effects
- PKA increase PDE4d activity
- PKA and PKG increase PDE5 activity

Question 9

Transducin (Gαt)

Answer 9

• activate GPCR rhodopsin by light = activate Gαt
• turns on PDE6 (cGMP selective phosphodiesterase)
• less cGMP = close cation channels = hyperpolarize

Question 10

Gαi/o Effector Pathways

Answer 10

Gαi/o inactivated by PTX
- suppress of N/L type Ca2+ channel opening
- activation of PLCβ

Question 11

Activation of Rho Signalling

Answer 11

• Gα12/13 activates p115-RhoGEF (PDZ-RhoGEF, LARG)
• activates Rho GTPase –> cytoskeletal changes

Gαq/11 activates p63RhoGEF

Question 12

Gαq Effector Pathways

Answer 12

Gq activates phospholipase Cβ (PLCβ)
- hydrolyzes PIP2 into DAG and IP3

DAG –> protein kinase C (PKC)
IP3 –> release of Ca2+ from SR/ER –> activates PKC

Question 13

G Protein Effects on Intracellular Calcium

Answer 13

PLCβ –> IP3 –> release Ca2+
cyclic nucleotide-gated ion channel –> Ca2+ influx
PKA phosphorylates Ca2+ channel –> Ca2+ influx

Gβγ inhibits voltage-gated Ca2+ channels

Question 14

Cytosolic Calcium Levels

Answer 14

Prolong elevation of cytosolic Ca2+ = toxic
- Na+/Ca2+ Exchanger (pumps Ca2+ out of cytoplasm)
- SERCA (return Ca2+ to ER with ATP)
- PMCA (remove Ca2+ from cell with ATP)