L2 - Introduction to cell signalling II Flashcards
What are the 3 general strategies used to transfer information across the PM?
Intracellular receptors
• Membrane-permeable small hydrophobic signals diffuse passively
Ion-coupled channels
• Signals bind to & regulate opening of membrane channels
Transmembrane receptors
• Signals bind to TM receptors - activation of intracellular enzymes
Intracellular receptors
Bound by small hydrophobic molecules that can pass through the hydrophobic membrane
Bind to nuclear receptors
Cellular response = relatively slow
How do steroid hormones work?
- Formation of a complex ligand-receptor
- Upon binding of the ligand, receptor becomes active
- Hormone-receptor complex translocates into the nucleus where it acts as a transcription factor turning on testosterone-responsive genes
Intracellular receptors
What can you call ion-coupled channels?
Ligand-gated ion channels
Ion-coupled channels
Ion-channel-coupled receptors
What are ligand-gated ion channels?
Transmembrane pores formed of multiple protein subunits
Key role in synaptic signalling: covert chemical signal (neurotransmitter) into electric signal (ion flow)
Cellular response = very rapid (milliseconds)
How do ligand-gated ion channels work?
- Ligand binds to extracellular part of channel
- Induces a conformational change of the receptor – opens the ion channels
- Results in a flow of small molecules such as ions across the cell membrane
Cell surface receptors
Most diverse & most commonly used
Most signal molecules are large and/or hydrophilic & cannot cross the PM so must bind to receptors
Each cell expresses a repertoire of TM receptors
What are transmembrane receptors?
Sensors that detect extracellular stimuli – act like a molecular switch
Cell surface receptor structure
EC ligand-binding site: highly specific & sensitive
1 or more hydrophobic transmembrane domains
IC portion of receptor coupled to downstream signalling
Receptor transduces EC signal into IC signalling cascade
2 common strategies used to transfer signals in cell surface receptors
Receptor conformational changes
Receptor clustering
3 main classes of transmembrane receptors
GPCR
Enzyme-linked receptor
Cytokine receptor
What are GPCRs?
7 transmembrane protein
Coupled with large protein G
Largest family of cell surface receptors in eukaryotes
Many functions: smell, taste, sight, endocrine function, neuronal communication
Many drugs act through GPCRs
What are enzyme-linked receptors?
The receptor has catalytic activity
What are cytokine receptors?
Receptor has no catalytic activity
Coupled to an intracellular enzyme
Process of GPCRs
Binding of ligand to EC domain induces a conformational change of receptor allowing its cytosolic domain to bind to & activate the G protein
G protein acts as an on/off switch: if GTP is bound to the G protein it is active
What are G proteins composed of?
3 subunits
• Alpha - binds GDP & GTP
• Beta & Gamma
2 lipid anchors
Activation of G proteins
- G protein is inactive & binds GDP
- GPCR binds its ligand & induces a change of conformation
- Alpha subunit binds GTP instead of GDP & becomes active
- Alpha subunit dissociates from beta & gamma & migrates to target protein
- Target protein activated
- 2nd messenger produced
Deactivation of G proteins
- Alpha subunit hydrolyses GTP by GTPase activity
- Alpha dissociates from target protein
- Alpha re-associates with beta & gamma
Example of GPCR signalling
Epinephrine receptor
Response to stress
Adenylyl cyclase produces cAMP which is the second messenger
Acts on multiple effectors
Allows person to run faster
Abnormal G-protein signalling can be caused by…
Cholera bacterial toxins
Gene mutations
Single nucleotide polymorphisms
How do cholera bacterial toxins cause abnormal G-protein signalling?
Binds to alpha subunit inactivates GTPase activity
Leads to defective signal termination with persistent elevated levels of cAMP & effector activity (loss of salt from epithelial cells of the intestine)
What types of gene mutations cause abnormal G-protein signalling?
Loss of function
• Signalling in response to the corresponding agonist
• Receptor becomes resistant to signal
Gain of function
• Leads to constitutive, agonist-independent activation of signalling
Mis-folding of GPCR
• Receptor retains its function but located in parts of cell where function in inappropriate
How do single nucleotide polymorphisms cause abnormal G-protein signalling?
Impact of SNPs on individual variations in response to drug as GPCRs mediate the therapeutic effects of more than 30% of FDA-approved drugs
What are receptor tyrosine kinases (RTKs)?
Single transmembrane span receptors
An enzyme coupled receptor
Extracellular side – ligand binding site
Cytoplasmic side – Tyr kinase domain & tyrosine’s
2 forms:
– Monomeric inactive
– Dimeric active
A RTK can trigger multiple signal transduction pathways at once
Autophosphorylation increases the catalytic efficiency of the receptor & provides binding sites for the assembly of downstream signalling complexes
RTK activation steps
- A bivalent ligand binds simultaneously to 2 RTKs inducing a dimerisation of monomeric RTKs
- Dimerisation activates Tyr kinase activity
- Autophosphorylation of Tyr domains
- RTKs interact with & activate proteins
- Cellular response
Structure of RTKs
EC domain
• Highly variable binding
• Recognition of various signals
TM domain
• Small
• Hydrophobic
IC domain
• Common: Tyr kinase domain
• Some kinase domains are interrupted by inserts
What is the EGF receptor?
Receptor for Epidermal Growth Factor (EGF)
Member of the ErbB family involved in angiogenesis, apoptosis, cell proliferation & metastasis
Activation of the EGF-R
Activation of the EGF-R by EGF
Phosphorylation of tyrosine of cytoplasmic proteins
Signals to the cell to proliferate & grow
Deregulation of EGF-R in cancer
Don’t need a ligand to activate the receptor in cancer cells
1 or more members of the family of EGRF genes are overexpressed or deregulated in most epithelial tumours by: – Activating mutation – Increase gene copy number – Overexpression – Truncation of receptor EC domain
Cytokine receptors: JAK-STAT signalling
Large family of receptors
Activated by cytokines (interferon & interleukins) or growth factors (prolactine & EPO)
Control synthesis & release of inflammatory mediators
Receptor has no catalytic activity but associated with kinase
What are the 3 main components of the JAK-STAT signalling pathway?
Receptor
Janus Kinases (JAK)
STATs
Receptors in the JAK-STAT signalling pathway
Binds to cytokine or growth factor
Receptor forms a dimer
Janus Kinases (JAK) in the JAK-STAT signalling pathway
‘Just Another Kinase’
Dimeric receptors associate with Janus Tyrosine Kinases (JAKs) & activate JAKs which:
• Cross phosphorylate each other
• Phosphorylate & activate R
• PO4- creates a docking site for SH2 domains of transcription regulators STATs
• JAKs phosphorylate & activate STATs
STATs in the JAK-STAT signalling pathway
Transcription regulators STAT: Signal Transducers & Activators of Transcription
Dissociate from the receptor & dimerise
Dimer migrates to the nucleus where it binds to promoter region & activates transcription of target genes
JAK-STAT signalling: example of leptin
What is leptin?
Satiety hormone – feeling of being full
Is produced by adipose (fat) tissue
Reduces food intake, controls metabolism & body weight
JAK-STAT signalling: example of leptin
The leptin receptor
Cytokine receptor in brain satiety centre
Binding of leptin to its receptor activates JAK proteins that phosphorylate tyrosine residues on the leptin receptor, allowing for the recruitment & phosphorylation of STATs
STAT proteins dimerise, translocate to the nucleus & act as transcription factors stimulating expression of genes that inhibit appetite
Feedback & adaptation at receptor level
Receptor sequestration
Receptor down regulation
Receptor inactivation
Inactivation of signalling protein
Production of inhibitory protein
Intracellular receptors
Response time & effects
HOURS
Alter gene transcription & protein expression
Ligand gated ion channels
Response time & effects
MILLISECONDS
Alter membrane potential
GPCRs
Response time & effects
SECONDS
Can mediate 2nd messengers or can directly activate the ion channels
Enzyme-linked receptors / cytokine receptors
Response time & effects
HOURS
Alter gene transcription & protein expression
What is transduction?
Signal relay, amplification & integration
Requires transducer molecules
Transducer molecules
Signalling proteins
• Kinases, Phosphatases, GTPases, Adaptors
Second messengers
• cAMP & cGMP, Lipids, Calcium, NO
Often act as molecular switches
Signal transduction cascade
Multistep process during which transducers: • Relay • Transduce • Amplify • Integrate • Regulate
Signal transduction cascade
TRANSDUCTION
At each step, signal transduced into a different form, commonly protein conformational change
Signal transduction cascade
AMPLIFICATION
Signal amplified by activating multiple copies of next component in pathway
Signal transduction cascade
INTEGRATION
Coordinate, distribute, regulate signal
Allow fine tuning of the response & contribute to response specificity
Signal transduction cascade
REGULATION
Many signalling pathways are regulated by post-translational modifications
Phosphorylation, ubiquitination & acetylation
Phosphorylation cascade
Reversible molecular switches
- Relay molecule activates kinase 1
- Active kinase adds phosphate to next kinase / effector in line, activating or inhibiting it
- Phosphatase removes PO4-, deactivating the kinase which can be reused – no need for protein synthesis
- Signal is amplified at each stage
Adaptor proteins
Have no intrinsic enzymatic activity
Usually contain several protein-binding domains or modules
These modules or motifs mediate intermolecular interactions between receptors & specific target proteins
How do adaptor proteins mediate intermolecular interactions between receptors & specific target proteins?
Act as docking proteins that link receptors with other effector proteins
Scaffold the organisation of large protein complexes between protein-binding partners
Dictate:
• Specificity of interactions between binding partners
• Subcellular localisation of binding partners
Integrate signals from different pathways (cross-talks) & coordinate responses
Adaptor proteins regulate cell signalling in a spatial & temporal manner
Adaptor protein Grb2 recruitment to EGFR-phospho-Tyr activates Ras & MAPK signalling cascade cascade in response to mitogens
- EGF binds to the receptor activating TK
- Autophosphorylation of the receptor
- SH2 binds phosphorylated tyrosine
- Sos binds SH2 by SH3 & converts GDP to GTP on Ras to activate it
- Ras phosphorylates RAF
- RAF phosphorylates MEK
- MEK phosphorylates MAP kinase which causes the response
What are second messengers?
Chemical relays from plasma membrane to cytoplasm - short lived
Small, non-protein, water-soluble molecules or ions
Greatly amplify the strength of a signal
2nd messenger information is encoded by local concentrations
Elevated concentration triggers rapid alteration of activity of cellular enzymes
Removed or degraded terminates cellular response
What are the 3 basic types of 2nd messenger molecules?
Hydrophobic molecules: membrane-associated
Hydrophilic molecules: water-soluble molecules in cytosol
Gases: can diffuse through cytosol & across membrane
cAMP signalling pathway
- Activation of GPCRs by hormone
- G-protein (G-alpha-stimulatory) activates adenylyl cyclase
- AC catalyses cAMP synthesis from ATP
- cAMP binds PKA regulatory subunits (Protein Kinase A – cAMP-dependent PK)
- Dissociation from the catalytic subunits
Catalytic subunits of PKA:
• FAST response: PKA-C phosphorylates enzyme in the cytoplasm (eg. glycogen phosphorylase)
• SLOW response: PKA-C enters the nucleus & phosphorylates transcription factors such as CREBP (cAMP response element binding protein). CREBP binds to promoters (CRE) & turns on gene expression
Inactivation of cAMP signalling pathway
- EC ligand/receptor binding reversible
- Deactivation of G protein by hydrolysis of GTP to GDP
- Phosphodiesterase breaks down cAMP: decrease in [cAMP] in cell
- Phosphatase removes phosphate on target effector
Other 2nd messengers
NO
cGMP
NO as a 2nd messenger
Gas, free radicals, diffuses across plasma membrane & affects nearby cells
Synthesised from arginine & oxygen by NO synthase
NO activates guanylyl cyclase which synthesises cGMP – toxic at high concentration
cGMP as a 2nd messenger
Synthesised from GTP using guanylyl cyclase in response to NO
Triggers cell response by activating cGMP-stimulated kinase or Protein Kinase G (PKG)
cGMP induces blood vessel relaxation (smooth muscles)
Phosphodiesterase breaks down cGMP
2nd messengers involved in the Inositol Phospholipid Pathway
Combination of several 2nd messengers: • DAG • PIP • IP3 • Ca2+
Inositol Phospholipid Pathway
- Activation of GPCRs
- G-protein (G-alpha-q) activates phospholipase C
- PLC hydrolyses PIP2 to DAG & IP3
- DAG (hydrophobic) recruits to membrane & activates PKC
- IP3 (soluble) diffuses through cytosol, binds receptors on ER triggering Ca2+ release into the cytosol
- Ca2+ binds to effector proteins:
– PKC: increases its activation
– Calmodulin – cytosolic Ca2+ binding protein
Cell response to the Inositol Phospholipid Pathway
Cell division
FAST: PKC phosphorylates effector protein
SLOW: calmodulin activates CAM-kinase (calmodulin dependent protein kinase) – CAM kinase phosphorylates transcription factors on the chromosome
