Park Flashcards
Describe full cycle of g-protein coupled signaling w/ respect to alpha/beta/gamma subunits
How many potential alpha subunits? Beta subunits? Gamma subunits?
20 alpha
6 beta
12 gamma
(in humans)
Useful terminology:
1. first messenger
2. receptor
3. transducer
4. primary effector
5. second messenger
6. secondary effector
- first messenger == ligand
- receptor == GPCR
- transducer == G-alpha
- primary effector == enzymes activated by G-alpha
- second messenger
- secondary effector
G-alpha signaling:
cAMP
G-alpha signaling:
phosphoinositol
G-alpha signaling:
arachidonic
The adenylate cyclase
pathway
Gs
Activation of GPCR
GDP/GTP exchange
Activation of AC
Synthesis of cAMP
Activation of PKA (Protein Kinase A)
PKA translocates to nucleus
PKA phosphorylates CREB
(cAMP response element
binding, a transcription
factor)
CREB recruits CBP (CREB
binding protein)
Both bind to CRE site
(cAMP resonse element)
Transcription is now activated
PLC Pathway
Gq
Function of PLC-beta
converted to:
diacylglycerol
–activates protein kinase C
IP3
– Releases Ca2+ of ER
Three types of ligands
Agonists
Ligands that shift the equilibrium in favor of the active state
(example: adrenalin, light).
Inverse agonists
Ligands that shift the equilibrium in favor of inactive states (example:
11-cis-retinal in rhodopsin, MPEP: mGluR5).
Neutral antagonists that do not affect the equilibrium, but can block
agonists to activate the GPCR
(example: carvedilol, a non-selective beta blocker).
[rhodopsin] on disc
(8 mM Rhodopsin on disc)
How does GPCR achieve “FAST” and
“SENSITIVE” responses?
High concentration
clustering of GPCR and its transducers for downstream effects. Largely amplified signal
Is G_alpha the only subunit used for signaling?
βγsubunit can directly regulate channel properties (SHORT RANGE SIGNAL)
beta_arrestin signaling
beta_arrestin signaling
GPCR activation/internalization by β-arrestin sequesters
signaling molecules of other pathways.
Complexity of GPCR signaling
- βγsubunit can directly regulate channel properties (short-
range signal). - β-arrestin regulates GPCR signaling in many ways: (1)
desensitization (blocking the binding of Gαto GPCR,
recruiting 2nd messenger-degrading enzymes), (2) GPCR
internalization (removing GPCR from the cell surface), and
(3) transducing other signaling such as MAPK pathway. - GPCRs serve as a scaffold of many signaling molecules.
Common Lipid modifications
Acylation
(Gαsubunit)
myristic acid (myr)
palmitic acid (palm)
Prenylation
(Gγsubunit)
farnesol (fa)
geranylgeraniol (gg)
prenylation
The CAAX box motif signals postranslational prenylation
C = cysteine
A = aliphatic (nonpolar and hydrophobic) residue
X = any residue
if x = L (leucine), C will be geranylgeranylated
if X = not L, C will be farnesylated
Lipid-attachment of heterotrimeric G-proteins
Most Gαsubunits are associated with membrane via acylation.
Gβγsubunit is always associated with membrane via Gγ-prenylation.
G protein activation (10 steps)
- GPCR becomes activated by an agonist
- The activation changes the structure of the GPCR (outward movement of TM5-
TM6) - GPCR interacts with G protein α-subunit carrying GDP
- The interaction induces a major displacement of the α-helical domain of Gα,
which reveals the nucleotide-binding pocket and causes an exchange of GDP
with GTP - The Gαand Gβγsubunits dissociate
- The activated GPCR acts as an enzyme (GEF), and turns over many G protein α-
subunits - The Gαsubunit with GTP attached is the activator of a target enzyme
(amplification) - Activation of the target lasts as long as GTP is bound
- Activation terminates when GTP is hydrolyzed to GDP
- Hydrolysis is accelerated by a GTPase Activating Protein complex, termed GAP
Name 2 applications of GPCRs
Optogenetics (Channel Rhodopsin) and
Pharmacogenetics (DREADDs)
Which structure in G protein is flexible?
helical domain of G-alpha. Allows for exchange of GDP to GTP
