Transcript The G-Proteins - mustafaaltinisik.org.uk

Membrane Function
Signal Transduction
I. Introduction to Receptors &
Signal Transduction
The Players
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Signaling molecules
Receptors
G-proteins
Second messenger systems
Effector proteins
Signaling Molecules
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Neurotransmitters
Hormones
Growth factors
Drugs
Other nomenclature
 Ligand
 Agonist / Antagonist
Receptors
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Receptors are proteins associated with
cell membranes
Receptors “recognize” signaling
molecules by binding to them.
Binding of receptors by signaling
molecules ---> Cell behavior change
Figure 1: Overview of Signaling
Transmitters
Hormones

Growth
Factors
Hormones:
Steroids
Thyroid
Transmitters


Second
Tyrosine Messangers
Kinase
Protein Kinases
Ion
Channels
mRNA
Synthesis
Protein Synthesis
Neurotansmitters: Biogenic
Amines.
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Catecholamines
 Epinephrine
 Norepinephrine
 Dopamine
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Esters: Acetylcholine
Indolamines
 Histamine
 5-HT
Neurotransmitters: Peptides
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Substance P
Neuropeptide Y (NPY)
Enkephalins
Somatostatin
VIP
Neurotransmitters: Amino
Acids
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Excitatory
 Glutamate
 Aspartate
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Inhibitory
 -aminobutyric acid (GABA)
 Glycine
Neurotranmitters: Other
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Nitric Oxide
Arachadonic acid
Carbon Monoxide
PAF
Zinc
The G-Proteins
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Involved in most signaling processes
Link receptor proteins to effector
proteins.
Trimeric proteins composed of , ,
and -subunits.
Figure 2: G-Protein Cycling
A
R


A

GDP
A
R
R




GTP
(GTPase)
-Pi
A
GTP
R



GTP
Adenylate Cyclase
Phospholipase C
Ion Channels
Phospholipase A2
Phosphodiesterase


GDP
Functional G-Protein Units
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GTP-activated -subunit
 produce second messenger
 and/or opens ion channels.
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-complexes
 Initially thought to be inert.
 Probably not inert
 Exact role currently ill-defined.
Second messengers produced by
G-protein activation.
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Adenylate Cyclase
 cAMP
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Phospholipase C (PLC)
 Inositol triphosphate (IP3)
 Diacylglycerol (DAG)
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Ion Channel Activity
Families of G-proteins
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Unique structure of their -subunits.
 subunits appear to be similar across
families.
Main families:
 Gs
 Gi
 Gq
II. cAMP Second Messenger
System
Figure 3: Adenylate Cyclase
Ai
As
Adenylate
Cyclase
R1
R2
Gs
Gi
GTP
GTP
GDP
AMP
ATP-Mg++
PDE
GDP
cAMP
C
C
Protein
Reg
Reg
C
C
Protein Kinase A
(PKA)
PKA
Protein-P
Summary of Adenylate Cyclase
Activation
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Receptors which associate with Gs -type
G-protein
 Stimulates adenylate cyclase.
 Increases cAMP
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Receptors which associate with Gi -type
G-protein
 Inhibit adenylate cyclase.
 Decreases cAMP
Summary of cAMP action
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cAMP exerts its effect by activating
protein kinase A (PKA)
PKA phosphorylates proteins
 Enzymes, pumps, and channels
 Phosphorylation can either increase or
decrease activity depending on the protein.
Adenylate Cyclase
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Family of membrane spanning
enzymes.
Types I through IV have been well
characterized.
 Additional types probably exist.
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Types differ with respect to activity
modulation by other second messenger
systems
Adenylate Cyclase Activity and
Other Messenger Systems
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Kinases (PKA, PKC, other) can
phosphorylate adenylate cyclase in
some cells.
Binding of adenylate cyclase by:
 -subunits of other G-proteins
 Ca++/calmodulin complexes
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Allows other second messenger
systems to interact with cAMP system
III. The Phospholipase C Second
Messenger System:
IP3 and DAG
Figure 4: Phospholipase C
System
Ca++
A
PLC
R
Gq
DAG
PKC
PIP2
Protein
IP3
Protein-P
Endoplasmic Reticulum
Ca++
Summary of the Phospholipase C
Messengers
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Agonist binds receptor
Occupied Receptor ---> activation of
PLC (Gq -mediated)
PLC Produces second messengers:
IP3 and DAG
PLC activation associated with
Ca++-channel activation
Action of IP3
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IP3 binds to IP3-receptors on the
endoplasmic reticulum
Releases intracellular Ca++ stores.
Action of DAG
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Remains membrane associated.
Activates Protein kinase C (PKC) which
translocates from the cytosol to the
membrane
Activated PKC phosphorylates other
proteins and alters their function state.
PLC System and Calcium
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PLC causes the IP3-mediated Calcium
PLC also causes the influx of Ca++.
Ca++ binds one of a family of Ca++
binding proteins (calmodulin).
Ca++/calmodulin complex
 binds to yet other proteins and changes
their functional activity.
IV. Guanylate Cyclase: cGMP
and Nitric Oxide As Second
Messengers
Figure 5: Nitric Oxide and
cGMP
Intracellular
Ca++ Stores
NO
Synthetase
Ca++
Ca++
Membrane Bound
Guanylate Cyclase
C.M.
Ca++
NO
GTP
Soluble
Guanylate Cyclase
NO
+
Citrulline GTP
cGMP
PDE
Arginine
Ion Channels
cGMP-Dependent PK
PDEase Activity
GMP
NO is Membrane Soluble.
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Diffusion to nearby cells
Increase cGMP levels in nearby cells
Vascular endothelial cells and nearby
smooth muscle cells.
V. SIGNALING BY
ACETYLCHOLINE
Acetylcholine As a
Neurotransmitter
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Both the central and peripheral nervous
systems.
Binds two broad classes of receptors:
 Nicotinic receptors
 Muscarinic receptors.
Nicotinic Receptor Features
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Composed of 5 subunits:
 2 , ,  and d.
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Subunits are arranged to form a central
cavity that extends across the
membrane.
Nicotinic receptors are also channels
ACh-binding opens gates and allows
ion fluxes across the channel
Figure 6: Nicotinic Receptor
Channel
Agonist
Binding
Site
Gate
Subclasses of Nicotinic Receptors
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Skeletal muscle (N1 or Nm)
 Unique  and  subunits
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Autonomic ganglia (N2 or Ng).
Both N1 and N2 are gene-product
families not single receptor types.
Other Ligand-Gated Channels
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Structural and sequence similarity to
nicotinic receptors.
Example agonists for these channels
include:
 Serotonin (5-HT)
 Glutamate
 GABA
 Glycine
Muscarinic receptors
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Muscarinic receptors are not channels.
Operate through G-proteins to alter
second messenger systems.
5 muscarinic subtypes have been cloned
and sequenced (M1, M2, M3, M4, M5).
Grouping Muscarinic Receptors
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M1, M3, and M5 receptors: Activate
Phospholipase C through Gq.
 PLC activation ---> increased IP3 -->
increased intracellular Ca++
 Increased intracellular Ca++ --->
Activation of Ca++-sensitive K+ & Clchannels.
Grouping Muscarinic Receptors
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M2 and M4 receptors
 Gi -coupled inhibition of adenylate cyclase
 Go or Gi -coupled regulation of certain
Ca++ & K+ channels.
VI. Signaling by Epinephrine
and Norepinephrine and
Coupling Through Adrenergic
Receptors
Three Families of Adrenergic
Receptors:
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 -receptors: Three subtypes 1, 2, and
3.
1 -receptors: Three subtypes 1A , 1B ,
and 1C
2 -receptors: Three subtypes 2A , 2B ,
and 2C
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All adrenergic receptors appear
to be coupled to cellular
processes through G-proteins
Occupation of  - Adrenergic
Receptors
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Gs-mediated stimulation of adenylate
cyclase
Increased cAMP
Increased PKA activity.
Occupation of 1 -Adrenergic
Receptors
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Mechanistic details sketchy
Possibly Gq-mediated PLC activation
 Increases IP3 and DAG for some subtypes
(1B)?
 Activates Ca++-channels for other subtypes
(1A)?
Occupation of 2 -Adrenergic
Receptors
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Gi -mediated inhibition of adenylate
cyclase.
Decreased cAMP
Decreased PKA activity.