signalling at cell surface 2 april 2007. receptors
TRANSCRIPT
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.
Table 20-1. Characteristic Properties of Principal Types of Mammalian
Hormones
Property Steroids Thyroxine Peptides and
Proteins
Catecholamines
Feedback
regulation of
synthesis
Yes Yes Yes Yes
Storage of
preformed
hormone
Very little Several weeks One day Several days, in
adrenal medulla
Mechanism
of secretion
Diffusion through
plasma membrane
Proteolysis of
thyroglobulin
Exocytosis of
storage vesicles
Exocytosis of storage
vesicles
Binding to
plasma
proteins
Yes Yes Rarely No
Lifetime in
blood
plasma
Hours Days Minutes Seconds
Time course
of action
Hours to days Days Minutes to hours Seconds or less
Receptors Cytosolic or nuclear Nuclear Plasma
membrane
Plasma membrane
Mechanism
of action
Receptor-hormone
complex controls
transcription and
stability of mRNAs
Receptor-hormone
complex controls
transcription and
stability of mRNAs
Hormone binding
triggers synthesis
of cystolic
second
messengers or
protein kinase
activity
Hormone binding
causes change in
membrane potential or
triggers synthesis of
cystolic second
messengers
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
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)
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
• 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
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
GDP
GTP
+
GTP
GDP
Inactiveeffector
Activeeffector
Pi
GTP GDP
Agonist-receptor complex
Activeeffector
The activation/deactivation cycle of G proteins
The activation/deactivation cycle of G proteins
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
OUT
ING protein
adenylatecyclase
cAMP
adrenalin
MEM BRANE
adrenalinreceptor
firstmessenger
receptor
transducer
amplifier
secondmessenger
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