Protein Laboratory, University of Copenhagen

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Signaling at the Cell Surface
Vladimir Berezin
Protein Laboratory Institute of Molecular
Pathology University of Copenhagen
Protein Laboratory, University of Copenhagen
http//plab.ku.dk/berezin/index.html
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What is Signal Transduction?
The conversion of a signal from one type to
another (e.g. chemical to electrical, electrical
to second messenger pathway, extracellular to
intracellular)
Signal-response relationship
Protein Laboratory, University of Copenhagen
The conversion of signals into cellular responses
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Extracellular Signals (e.g. hormones, cytokines,
ECM-molecules, CAM, neurotransmitters, UV)
Membrane Cell Receptors
Cytosolic Receptors
Membrane-to-Nucleus Signaling Modules (signal
transduction cascades)
Nuclear Receptors
Transcription Factors (e.g. c-Fos, c-Jun, CREB,
elk1, Hes-1)
Remodelling of the Cytoskeleton
Protein Laboratory, University of Copenhagen
Regulation of Gene Expression
Cellular Response (e.g. adhesion, spreading,
motility, cell cycle progression, cell
differentiation, apoptosis, other changes in
cellular phenotype)
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How do signaling molecules reach the cell?
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(C) AUTOCRINE
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Receptors
-Ligand-binding specificity
-Effector specificity (signaling pathways)
Note
-Different receptors with different effector
specificity can bind the same ligand
-In striated muscle (contraction) ion channel
AChR
-In heart muscle (slow the rate of contraction
G-proteins
-In pancreatic acinar cells (secretion)
exocytosis of secretory granules with digestive
enzymes
-Different receptors with different binding
specificity may have the same effector
specificity
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Receptors
Sensitivity to external signals
-Signal molecule concentration
-Binding affinity
RL
1
____
________

RT
1 Kd/L
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-Number of receptor molecules
-Number of receptor molecules occupied by a
ligand to trigger cellular response (threshold)
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Receptors
-Synthetic analogues, agonists, mimetics.
antagonists
Agonist promotes relaxation of bronchial smooth
muscle, 10 x stronger than EP
Antagonist (beta-blocker) binds with high
affinity to cardiac muscle EP-R, slow heart
contraction (arrhytmias, angina)
-Identification
Protein Laboratory, University of Copenhagen
-Purification and cloning
-Functional expression assay
-Genomics studies and cloning
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Receptor-mediated signal transduction
Basic principles
-Many receptors, few second messengers
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Receptor-mediated signal transduction
Basic principles
-Switch proteins
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Guanine nucleotide-exchange factor (GEF)
GTPase-accelerating protein (GAP)
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Receptor-mediated signal transduction
Basic principles
-Protein kinases
-Protein phosphatases
-Compartmentalization by clustering (lipid
rafts/DRD/caveolae) and scaffolding
Protein Laboratory, University of Copenhagen
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Major receptor classes
1. Trimeric G protein-linked/coupled receptors
(GPCRs) (e.g. glucagon-, serotonin-,
epinephrine-receptors)
2. Ion-channel receptors (ligand-gated
ion-channels, e.g. the acetylcholine receptor)
3. Receptors lacking intrinsic catalytic activity
but directly associated with cytosolic protein
tyrosine kinases
4. Receptors with intrinsic enzymatic
activity (e.g. guanylate cyclase activity,
protein phosphatase, serine/threonine kinase or
tyrosine kinase activiy)
Protein Laboratory, University of Copenhagen
5. Cell adhesion molecules
6. Intracellular and nuclear receptors
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GPCRs

• Among membrane-bound receptors,
• the G protein-coupled receptors
• (GPCRs) are the most diverse.
• In vertebrates, this family contains
• 1000 2000 members (gt1 of the genome).
• -GPCRs have been very successful during
• evolution, being capable of transducing
• messages as different as photons, organic
• odorants, necleotides, peptides, lipids
• and proteins.
• GPCRs have a common central core,
• composed of 7 transmembrane helical
• domains.
• -The fine-tuning of coupling of the receptor
• to G proteins is regulated by splicing,

Protein Laboratory, University of Copenhagen
Illustration of the central core of rhodopsin.
The core is viewed from the cytoplasm.
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GPCRs
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Signaling via GPCRs
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GPCRs that regulate adenylyl cyclase
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Signaling from GPCRs
Regulation of glycogen metabolism in liver and in
muscle cells
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Signal amplification
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GPCRs that regulate ion channels
Muscarinic AChR-induced opening of K channels in
heart muscle cells (ACh slows the rate of heart
muscle contraction)
Protein Laboratory, University of Copenhagen
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GPCRs that activate PLC
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GPCRs that activate PLC
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Regulation/termination of signaling from GPCR
Rapid
-Receptor-ligand affinity is decreased by GTP-Gsa
-GTP hydrolysis
-cAMP-PDE
-heterologous (by PKA)
-homologous (by BARK, ß-arrestin)
-Desensitization
Slow
-endocytosis/recycling
Anchoring to specific subcellular regions
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GPCRs and gene transcription
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Main conclusions
Binding of extracellular signaling molecules to
cell surface receptors triggers
intracellular signal transduction pathways that
ultimately modulate cellular metabolism,
function, or gene expression.
Signals from one cell can act on distant cells
(endocrine), nearby cells or on the same cell.
Trimeric G proteins transduce signals from
coupled cell surface receptors to associated
effector proteins, which are either enzymes that
form effector proteins or cation channels
proteins.
An external signal is amplified downstream from a
cell surface receptor.
Protein Laboratory, University of Copenhagen
Activated by GPCRs enzymes affect gene
transcription either by releasing or
activating transcription factors.