Transcript Cell signalling - Bilkent University

Signalling at Cell Surface
2 April 2007
Receptors
Classification of receptors
• Intracellular receptors (for lipid soluble
messengers)
• function in the nucleus as transcription factors to
alter the rate of transcription of particular genes.
• Plasma membrane receptors (for lipid
insoluble messengers)
• Receptors function as ion channels
• receptors function as enzymes or are closely
associated with cytoplasmic enzymes
• receptors that activate G proteins which in turn act
upon effector proteins, either ion channels or
enzymes, in the plasma membrane.
Cell Surface Receptors
• May work both fast and slow
• Always use “second messengers”
Table 20-1. Characteristic Properties of Principal Types of Mammalian
Hormones
Property
Steroids
Thyroxine
Peptides and
Catecholamines
Proteins
Feedback
Yes
Yes
Yes
Yes
Very little
Several weeks
One day
Several days, in
regulation of
synthesis
Storage of
preformed
adrenal medulla
hormone
Mechanism
Diffusion through
Proteolysis of
Exocytosis of
Exocytosis of storage
of secretion
plasma membrane
thyroglobulin
storage vesicles
vesicles
Binding to
Yes
Yes
Rarely
No
Hours
Days
Minutes
Seconds
Hours to days
Days
Minutes to hours
Seconds or less
Cytosolic or nuclear
Nuclear
Plasma
Plasma membrane
plasma
proteins
Lifetime in
blood
plasma
Time course
of action
Receptors
membrane
Mechanism
Receptor-hormone
Receptor-hormone
Hormone binding
Hormone binding
of action
complex controls
complex controls
triggers synthesis
causes change in
transcription and
transcription and
of cystolic
membrane potential or
stability of mRNAs
stability of mRNAs
second
triggers synthesis of
messengers or
cystolic second
protein kinase
messengers
activity
Cell-Surface Receptors Belong to
Four Major Classes
• G protein coupled receptors : epinephrine,
serotonin, and glucagon.
• Ion-channel receptors: acetylcholine receptor
at the nerve-muscle junction.
• Tyrosine kinase linked receptors: cytokines,
interferons, and human growth factor.
• Receptors with intrinsic enzymatic activity
Four classes of ligand-triggered cellsurface receptors
RECEPTOR ION CHANNELS
•
•
•
•
multi-subunit, transmembrane protein complexes
complex is both the receptor and ion channel
stimuli: chemical, stretch or voltage
stimulus induces conformational change to open or close
ion channel
• two types:
1) ligand-gated ion channel
2) voltage-gated ion channel
LIGAND-GATED ION CHANNELS
• chemical stimuli bind to receptor and open or
close ion channel
• stimuli can be extracellular or intracellular
EXTRACELLULAR STIMULI: (neurotransmitters)
– e.g. acetylcholine, dopamine, GABA, glutamate
INTRACELLULAR STIMULI: (second messengers)
– e.g. IP3, cAMP, cGMP, Ca2+
LIGAND-GATED ION CHANNEL AT
THE SYNAPSE
• occurs at gap (synaspe)
between nerve and target cell
• acetylcholine (ACh) released
into synapse
• ACh binds to ion channel on
target cell, opens channel,
influx of Na+
• enzyme acetylcholinesterase
released into synapse to
breakdown ACh
Na+
Na+
Na+
ACETYLCHOLINE ANTAGONISTS
• very potent neurotoxins
• bind to receptor and prevent opening of Na+
channel
– e.g. cobratoxin from Indian cobra
–
atropine from deadly nightshade
–
S. American arrow poison (curare) - very
fast acting so shot animals don’t run too far
VOLTAGE GATED ION
CHANNELS
• ion channel undergoes conformational
change folllowing electrical stimulus
• this “depolarization” opens the channel
– leads to flow of Na+ into cell
– constitutes an “action potential”
• channel recloses
Signaling pathways downstream from G protein
coupled receptors (GPCRs) and receptor tyrosine
kinases (RTKs)
Structural formulas of four common
intracellular second messengers.
Intracellular proteins
• Two groups of evolutionary conserved
proteins function in signal transduction
• 1. GTPase switch proteins
– Conversion from GDP bound inactive state to
GTP-bound active state is mediated by guanine
nucleotide exchange factors (GEFs)
– Intrinsic GTPase activity hydrolyzes bound GTP
to GDP + Pi
• GTP hydrolysis is accelerated by GTPase
accelerating protein (GAPs)
• Two classes of GTPase switch proteins:
– Trimeric (large) G proteins
• Directly bind to receptors
– Monomeric (small) G proteins
• Linked to receptors via adapter proteins and GEFs
Common intracellular signaling
proteins
• 2. Protein kinases and phosphatases
– Human genome encodes 500 PKs and 100
PPs
– Two types of PK
• Those that P* OH group on Tyr residue
• Those that P* OH group on Ser or Thr residues
– PK is activated
• By other kinases
• By direct binding to other proteins
• By second messengers
Regulation of signaling
• External signal decreases
– Degradation of second mesenger
• Desensitization to prolonged signaling
– Receptor endocytosis
• Modulation of receptor activity
– Phosphorylation
– Binding to other proteins
G Protein-Coupled Receptors
• A very large family of receptors coupled to
trimeric G proteins
• Activate or inhibit adenylyl cyclase
• All have seven membrane spanning region
• Ligands include:
– Hormones, neurotransmitters, light activated receptors
(rhodopsins), thousands of odorant receptors
GPCRs and G proteins are involved in
the regulation of many important
physiological functions
• Signal transducing G protein has 3
subunits
– G, Gß and G
• G is the GTPase switch protein and
modulates the activity of an effector
protein
• Effector proteins are either membrane
bound ion channels or enzymes generating
second messengers
• GPCR-mediated dissociation of trimeric G
proteins has been demonstarted in
fluorescence energy transfer experiments
The activation/deactivation cycle
of G proteins
 
Agonist-receptor
complex
GDP 
GTP
GDP
+

GTP
GDP







Active
effector
Inactive
effector
Pi

GTP
Active
effector
G proteins can be linked to:
• adenylate cyclase
– produces cyclic AMP (cAMP)
• guanyl cyclase
– produces cyclic GMP (cGMP)
• phospholipase C
– produces inositol trisphosphate (IP3)
and diacyl glycerol (DAG)
• ion channels
G-ProteinActivated
Enzymes
first
messenger
adrenalin
adrenalin
receptor
receptor
OUT
MEMBRANE
IN
transducer
amplifier
G protein
adenylate
cyclase
cAMP
second
messenger
Activity of
 subunits
• Activation of
K+ Channels
G-Protein-Activated Enzymes
• Generate new molecules - “second messengers
G proteins and cAMP
cAMP vs PKA
cAMP and gene transcription
Epinephrine case
• Mediates body’s response to stress, when all
tissues need glucose and fatty acids to produce
ATP
• ß-adrenergic receptors
– Heart muscle: contraction
– Smooth muscle cells of intestine: relax
• 2-adrenergic receptors
– Smooth muscle cells of endothelium, skin, kidney and
intestine: constrict
• ß1 and ß2 adrenergic receptors are coupled to
stimulatory G protein (Gs)
– Actvates adenylyl cyclase
• 1 adrenergic receptor is coupled to inhibitory G
protein (Gi)
– Inhibits adenylyl cyclase
• 2 adrenergic receptor is coupled to Gq that
activates another effector enzyme
• Bacterial toxins
– Vibrio cholera
• Catalyzes chemical modification of Gs that
prevents hydrolysis of GTP to GDP
– Active state
– Bordetella pertussis
• Catalyzes chemical modification of Gi that
prevents release of GDP
– Inactive state
• Critical domain of GPCR resides in C3 loop
according to chimeric receptor expression
experiments
• Differential modulation of adenylyl cyclase
• Different hormone-receptor complexes
modulate the activity of the same effector
molecule
– In liver glucagon and epinephrine bind to
different receptors but activate the same Gs:
same metabolic responses