basics of cell signalling Flashcards
What is signal transduction?
Cell surface receptors convert extracellular signals into intracellular signals
leads to metabolic changes allowing the cell to adapt to changes
4 types of cell signalling?
Paracrine- to target cells nearby
Autocrine- signal released from cell to its own receptors
Endocrine- go to target cells that are further away
Direct contact
Synaptic
Example of signal transduction?
Epinephrine (adrenaline) binding to receptor
causes adenylyl cyclase to convert ATP to cAMP
glycogen converted to glucose
What signals can cells receive beyond plasma membrane?
Hormones Neurotransmitters Antigens Light Touch Pheromone (chemical)
Where can signals originate from?
Hormones- act at a distance
Growth factors- action is long lasting
Neurotransmitter (secreted close to target cells)
Pheromones (act on cells in different organism)
What is a pheromone?
A chemical factor secreted that triggers a social response in the members of the same species
What changes can these signals cause in a cell?
Differentiation
Antibody production
Growth in size/strength
Asexual/sexual cell division (mitosis/meiosis)
What are the 6 steps involved in cell-cell communication?
- Synthesis of signalling molecule
- Signalling molecule released from signalling cell by diffusion/exocytosis/cell-cell contact
- Signalling molecule transported to target cell
- Signal is detected by receptors on target cell (transduction)
- Receptor-signal complex triggers change in cell’s metabolism/function/development
- Signal is removed
What are effector proteins?
Cause response in cell by intracellular signalling molecules
Examples of effector proteins
- Metabolic enzyme- alters cell metabolism
- Transcription regulatory protein- alters gene expression
- Cytoskeleton protein- alters cell shape/movement
Direct contact
Signalling across gap junctions
Signalling cell with membrane bound signal molecule binds to target cell via gap junctions
Paracrine signalling
Signals released into extracellular space and acts locally on neighboring cells
Autocrine signalling
Signalling cell releases signal molecule that acts on the signalling cell itself
Endocrine signalling
endocrine cells secrete hormones into bloodstream
Synaptic signalling
Neurons transmit electrical signals along their axons
Release neurotransmitters at a synapse
synapse is far from neuron cell body
What happens once the signalling molecule has binded to receptor?
- A single response (glucagon)
- Variety of responses (adrenaline/epinephrine)
- Secondary messenger systems
What do protein kinases do?
Phosphorylate proteins by using a phosphate from ADP
What do protein phosphatases do?
Dephosphorylates proteins
What can phosphorylation of proteins do?
- Turns protein on/off
2. Activates/deactivates proteins in signalling transduction pathways
What are the 3 types of cell surface receptors?
- Ion-channel coupled receptors
- G-protein coupled receptors
- Enzyme coupled receptors
Ion-channel coupled receptors
Synaptic signalling betwen neurones mediated by neurotransmitters that open/close ion channels changing ion permeability of PM and excitability of post-synaptic target cell
G-protein coupled receptors
Trimeric GTP binding protein (G protein) mediates interaction between activated receptor and target cell
Enzyme-coupled receptors
Ligand binding site on outer surface of PM
enzyme binding site= inside- can act as an enzyme or associate directly with an enzyme
e.g target cell is activated when protein kinases phosphorylate proteins inside target cell
What to intracellular signalling proteins do inside target cell?
Relay signal (passing signal along) To activate effector protein= response
How is a signal processed inside cell? (passed along)
- IC signal protein relays signal to next signalling component
- Protein can act as a scaffold protein- bring 2/more signalling proteins together-bind-interact more quickly and efficiently
- Transduce signal from 1 form to another form
- Large amounts of small intracellular mediators are made= multiple amplifications= signalling cascade
Generally how does a GPCR/Enzyme coupled receptor cause response in target cell?
- Extracellular signal binds to GPCR/ECR (1st messenger)
- GPCR/ECR is activated
- Small intracellular mediators/secondary messengers e.g cAMP/Ca2+ are activated
- Alters conformation and behaviour of effector proteins
- Changes behaviour of cell
Structure of GPCR
7 transmembrane protein
G protein/trimeric GTP binding protein structure
Heterotrimeric
3 subunits- alpha, beta, gamma
G protein inactive state= GDP bound to alpha subunit
G protein active state= GTP bound to alpha subunit
Pathway of GPCR
- Extracellular signal binds to GPCR
- Conformational change of GPCR- alpha subunit is induced by GEF to release GDP bound to it
- GTP binds to alpha subunit
- Alpha subunit and beta-gamma subunit interact with its targets
Inactive state of G protein
alpha subunit is a GTPase
GTPase hydrolyses GTP into GDP when inactive
What depends on the time the G protein is active for?
Depends on how quickly GTPase on alpha subunit hydrolyses GTP
Why is the time that G protein is active for short?
GTPase activity is enhanced by RGS (regulator of G protein signalling)
How does GPCR affect cAMP production?
GPCR can increase/decrease cAMP production
How does GPCR increase cAMP production?
GPCR coupled to a stimulatory G protein (Gs)
This protein increases adenylyl cyclase= increases cAMP production
How does GPCR decrease cAMP production?
GPCR coupled to a inhibitory G protein (Gi)
inhibits adenylyl cyclase
decreases cAMP production
What pathway can GPCR activate?
Phospholipid signalling pathway by phopsholipase C
Phospholipid signalling pathway
- Ligand binds to GPCR (GTP bound to alpha subunit)= activated GPCR
- Phospholipase C is activated (membrane bound enzyme)
- PLC cleaves PIP2 into IP3
- IP3 leaves PM
- IP3 diffuses into cytosol into ER
- IP3 binds to IP3 receptors in ER
- Ligand gated ion channels release Ca2+ into cytosol (affect activity of target proteins e.g protein kinases and phosphatases)
Aftermath of phospholipase C pathway
Hydrolysis of PIP2= increase in Ca2+ concentration in cytosol of cell
other cleavage product of PIP2= diacylglycerol
What does diacylglycerol do?
Activates protein kinase C
phosphorylates target proteins
amplification of signal
Example of enzyme coupled receptors
Receptor tyrosine kinases
Enzyme coupled receptors characteristics
Transmembrane proteins
ligand binding domain= outer surface of PM
domain in cytosol= enzyme activity/ associated directly with an enzyme
Has 1 transmembrane segment= 1 subunit
Structure of receptor tyrosine kinases
2 receptor chains (inactive monomers)
Each receptor has a kinase domain
Within kinase domain- tyrosines
How are RTKs activated?
Ligands bind to binding site
Growth factor causes dimerization of receptor chains
RTK is activated
Dimer cross-phosphorylates tyrosines in kinase domain= TRANSAUTOPHOSPHORYLATION
How does cross-phosphorylation of tails of RTK activate receptor?
- Phosphorylation of tyrosines in kinase domain= increases kinase activity of enzyme
- Phosphorylation of tyrosines outside kinase domain creates DOCKING sites
What are docking sites?
Where specific intracellular signalling proteins bind to
Intracellular signalling proteins bind to specific phosphorylated sites on activated RTK
these signalling proteins can become phosphorylated= activated
Kinase cascade
Environmental stress activates mitogen activated proteins kinases(MAPK)
Protein kinases phosphorylate each other
MAP kinase is finally phosphorylated
Causes substrates to be made e.g transcriptional factors/cytosol proteins/cytoskeletal proteins
Substrates cause changes in cell- metabolism, cytoskeleton
How do kinase cascades cause amplification of signal/
Each protein kinase enzyme can act on many substrates
Large amount of final product
meaning larger response
What are scaffold proteins?
Proteins that simultaneously bind 2/more proteins
Enhance signalling efficiency and fidelity
These organise kinase cascade for efficiency
What are intracellular receptors?
Receptors in the cytoplasm
Ligand must pass through plasma membrane to reach receptor
Which hormones can diffuse through PM into cell and bind to receptor?
Steroid hormones
How many functional domains do steroid hormone receptors have?
- Hormone binding domain
- DNA binding domain
- Activating
Hormone binding domain of steroid hormone receptor
Hormone receptor complex forms in nucleus
Activated hormone-receptor complex binds to DNA Activates specific genes
Increasing/decrease production of proteins
DNA binding domain
High affinity to DNA
Affect protein production, regulating cell growth and division
Storing DNA inside the nucleus, DNA replication
What are Beta-adrenergic receptors?
GPCRs
Transmembrane proteins
Mediate critical sympathetic responses in the cardiovascular, pulmonary, metabolic, and central nervous systems.
What is the desensitization of Beta-adrenergic receptor?
The receptor decreases its response to an agonist (e.g epinephrine) at high concentration
What 2 proteins do desensitization of BAR involve?
- beta-adrenergic receptor kinase (BARK)
2. beta arrestin (BARR)
Process of desensitization of BAR
- Epinephrine binds to BAR
- Triggers dissociation of alpha from beta-gamma of G protein
- beta-gamma recruits BARK to membrane
- BARK phosphorylates ser residues at carboxyl terminus of receptor
- BARR binds to phosphorylated carboxy- terminal domain of receptor
- Receptor-arrestin enters cell by endocytosis
- In endocytic vesicle- arrestin is broken down and receptor is dephosphorylated and returned to cell’s surface
What is negative feedback of receptors?
Receptor activation= produces secondary messengers
Stimulates many kinases
Kinases phosphorylate C- terminal tail of receptor
This inhibits interactions of G proteins with receptors still bound to their ligands
Examples of these kinases =cAMP
What does cell signalling depend on?
- Specificity of protein-protein interactions
- Ability of ligand binding to induce a conformational change
- Modulates target specificity e.g transcription/translation
What are agonists?
Bind and activate cellular receptor e.g epinephrine
Isoproterenol agonizes β2-adrenergic receptors (used to treat asthma)
What are antagonists?
Counteract the effects of a natural compound by binding to the cellular receptor and blocking its action
Propranolol antagonizes β2-adrenergic receptors (controls blood pressure)
Types of G proteins and what they do
Gq,Gi,Gs
Gq= activates enzyme phospholipase C (found in membrane)- phospholipid pathway= increases Ca2+ in cell= changes electrical charge of cell= depolarisation
Gs= activates adenylyl cyclase= ATP to CAMP-moves out of cytoplasm and binds to protein kinase C=phosphorylates target proteins= trigger cellular response
Gi= inhibits adenylyl cyclase= to inactivate cells
Enzyme coupled receptors
2 domains-1=receptor that ligand binds to, other domain= enzyme (inside cell)- this enzyme is sually protein kinase-phosphorylates receptor domain
Receptor tyrosine kinase (ECR)
Ligand binds to receptor domain
2 chains dimerise
transautophosphorylation of chains= tyrosines are phosphorylated
triggers conformational change= high affinity binding sites for secondary messengers= trigger signalling pathway
What are nuclear receptors?
Located within the cell
So only available for lipophilic ligands (can diffuse through PM)
All of them act as ligand-activated transcription factors
Steroid receptors (homodimeric)
(–) Hormone – in the cytoplasm bound to heat shock protein (HSP) complexes
(+) Hormone – receptors translocate to the nucleus and bind to response elements
Heterodimeric nuclear receptors
Located exclusively in the nucleus.
(–) Hormone – repress transcription when present on their DNA-binding sites (epigenetic mechanisms)
(+) Hormone – conformational change – reverses repressing effects
Example of an RTK?
Insulin receptor
Insulin receptor mechanism
Plays a central role in the regulation of blood glucose levels and glucose homeostasis
1. Insulin binds the IR extracellular domain
2. IR intracellular domains autophosphorylate
allows the insulin response protein to bind (docking protein)
3. Activates glycogen synthase – induces the conversion of glucose to glycogen
4. Blood sugar levels decrease
Example of a GPCR
Glucagon receptor
Glucagon receptor mechanism
- Plays a central role in the regulation of blood glucose levels and glucose homeostasis
- Binding to receptors in muscle or liver cells induces the breakdown of glycogen to glucose
- Activates adenyl cyclase → elevates cAMP (secondary messenger) → activates protein kinase A (PKA) → cellular response
- blood sugar levels increase (glycogenolysis)
β2-adrenergic receptor (β2AR)
- A type of GPCR
Epinephrine (also known as adrenaline) is a hormone made in adrenal glands (pair of organs on top of kidneys).
Mediates stress response: mobilization of energy
binding to receptors in muscle or liver cells induces the breakdown of glycogen to glucose
Activates adenyl cyclase → elevates cAMP → activates protein kinase A (PKA) → cellular response
blood sugar levels increase
also – binding to receptors in adipose cells induces lipid hydrolysis
also – binding receptors in heart cells increases heart rate
How is β2AR and epinephrine of 1 signalling molecule having different effects?
- In the heart: generates Gαs and activates adenylyl cyclase → increases contraction strength
- In the smooth muscle of the intestine: generates Gαi and inhibits adenylyl cyclase → muscle relaxation
What does activation of intracellular signalling cascades do?
Amplify and integrate signals from receptors to enable… CONTROLLED RESPONSES to events
