Signalling Flashcards
What is signalling?
How a single cell responds to environmental changes
How does an extracellular signal affect cells?
the signal will not enter the cell, but it will induce changes within the cell
T or F: different signals will cause different cellular responses
true
What happens to a cell if there’s no incoming signals?
usually apoptosis
T or F: the same signal can cause different effects in different cells
true
What is an example of a signal?
neurotransmitters
hormones
What are the 2 basic ideas of how a cell can be changed?
- fast
2. slow
Describe the fast way to induce intracellular change
This occurs when extracellular signals change proteins that already exist
this results in a fast response
Describe the slow way to induce intracellular change
This occurs when extracellular signals alter gene expression
results in a slow response
What are the 3 basic steps of signalling pathways?
- a signalling cell sends the first messenger (signal ligand) to bind to a receptor on the plasma membrane of a second cell
- on the 2nd cell, a conformational change
sends the signal across the membrane to the cytoplasmic domain - one of two possible routes:
a. cytoplasmic signal cascade
b. a second messenger
What is the first messenger?
a signal ligand
What are the two possible routes that follow a conformational change causing a signal to be transduced across the membrane?
a cytoplasmic signal cascade
a second messenger
What generates the second messenger?
nearby effector enzymes in the cytoplasm
Where can the second messengers be located?
they can stay in the membrane or they can diffuse through the cytoplasm
What does the second messenger usually do?
binds and triggers cytoplasmic signalling (ex. a signal cascade)
What will the cytoplasmic domain of the receptor cause?
a change in a target protein (ex. phosphorylation)
What happens when a target protein changes conformation?
it will change the conformation of another downstream protein (either activating or inactivating it) - this is the signal cascade
the signal cascade will continue until the final protein influences cellular processes
When will the cytoplasmic signal cascade stop?
When the final protein influences cellular processes
What alters the conformations of signalling proteins?
kinases and phosphatases
What is the result of kinase and phosphatase altering the conformation of signalling proteins?
either the inactivation or activation within the cytoplasmic signal cascade
Approximately how many cellular proteins are subject to phosphorylation-dephosphorylation?
~1/3
What does kinase do?
phosphorylates proteins
What does phosphatase do?
dephosphorylates proteins
What are the 2 common types of receptors?
G-Protein-Coupled Receptors (GPCR)
Enzyme-coupled receptors (specifically RTKs)
What are GPCRs coupled to?
G-proteins
What is the largest superfamily of proteins encoded by animal genomes?
GPCRs
What is a GPCR?
a 7-pass transmembrane proteins
Where on a GPCR does a ligand bind? What does this cause?
binds to the extracellular side causing conformational changes on the cytosolic side
What side of the membrane will conformational changes occur after the ligand binds to a GPCR?
cytosolic side
How many times does GPCR pass through the membrane?
7
Describe the structure of GPCR
7-pass transmembrane protein
extracellular: 3 loops and the N-terminus bind the ligand
intracellular: 3 loops and the C-terminus bind to an associated G-protein
What part of a GPCR binds the ligand? What side of the membrane does this occur on?
the 3 loops and the N-terminal on the extracellular side
What part of a GPCR binds the G-protein? What side of the membrane does this occur on?
3 loops and the C-terminus on the intracellular side
How many subunits do the G-proteins that bind to GPCRs have? are they the same or different subunits? What does this make G-proteins?
they have 3 different subunits
therefore they are heterotrimeric
What are the 3 different subunits of G-proteins?
alpha
beta
gamma
In the GPCR pathway, how does the trimer G-protein start?
bound to GDP and inactive
What activates the G-proteins involved in the GPCR pathway?
ligand binding (a signal molecule) to the GPCR causes a conformational change in the GPCR that exchanges the GDP on GDP-bound G-protein for a GTP –> activating the G-protein
What happens when the G-protein is activated?
G-protein bound to GTP dissociates
How does the G-protein dissociate when it becomes active?
the beta and gamma subunits separate from the alpha subunit
Which subunits of the trimeric G-protein are lipid bound?
alpha and gamma
What happens after the G-protein dissociates?
Depending on the pathway, either activated beta and gamma will activate an effector
or
activated GTP-bound alpha will activate an effector
Where is the effector molecule located?
it is embedded in the membrane
Which effector molecule will we focus on for the GPCR pathway?
adenylyl cyclase
What happens when adenylyl cyclase (effector) is activated by GTP-bound alpha subunit?
adenylyl cyclase converts ATP into cAMP
What does adenylyl cyclase convert ATP into? What causes this to happen?
Adenylyl cyclase converts ATP into cAMP
happens when adenylyl cyclase is activated by the GTP-bound alpha subunit
What does cAMP stand for?
Cyclic AMP (Adenosine MonoPhosphate)
BRIEFLY state what the reaction of converting ATP into cAMP involves
removing 2 phosphates from ATP
cyclisizing the remaining phosphate to the ribose sugar
What happens when adenylyl cyclase is activated?
the GTP bound to the alpha subunit of the G-protein will be hydrolyzed to GDP, turning the G-protein off
What happens when the G-protein is turned off?
the alpha subunit again has affinity for the beta and gamma subunits
What does the alpha subunit of the G-protein regaining affinity for the beta and gamma subunits cause?
alpha subunit of the G-protein dissociates from the adenlyl cyclase and binds to the beta and gamma subunits to reform the G-protein complex and the cycle can begin again
What does cAMP activate?
Protein Kinase A (PKA)
How does cAMP activate PKA?
cAMP binds to the PKA regulatory subunits of the PKA tetramer which causes them to dissociate from the catalytic subunits
the free catalytic subunits can now go around and phosphorylate things
Where on the PKA does cAMP bind to?
on the PKA regulatory subunits of the PKA tetramer
What is released from PKA when cAMP binds?
the catalytic subunits of the PKA tetramer are released to go phosphorylate a bunch of stuff
Give some examples of things that can be phosphorylated after cAMP activates PKA
triglyceride lipase –> fatty acid formation
glycogen synthase –> glycogen formation
Nucleus –> DNA synthesis, differentiation, RNA synthesis
ER –> protein synthesis
phosphorylase –> glycogen breakdown
Microtubules –> assembly/disassembly
Describe an example of what happens when the active PKA travels into the nucleus
it will phosphorylate the transcription factor CREB, allowing it to bind to the promoter and help in transcription initiation
Describe the effect of signal amplification in regards to the GPCR pathway
a SINGLE ligand bound to a GPCR can activate SEVERAL G-protein complexes
Each G-protein complex activates an adenylyl cyclase
Each cAMP molecule activates several kinases
Each kinase activates several targets in each step of the cascade
T or F: a large number of ligand molecules is required to produce a large cellular response
FALSE!
a small number of ligands can create a large effect
What is it called when the GPCR signal is stopped?
Desensitization
What is one way the GPCR cycle is stopped?
When a GRK (kinase) phosphorylates the GPCR and an arrestin causes endocytosis of the receptor to stop the signal
What does GRK do?
it phosphorylates the GPCR when the cycle needs to be stopped
What does the phosphorylation of the GPCR by GRK cause?
A protein called arrestin comes in to cause endocytosis of the GPCR and arrest the signal
Describe arrestin
A protein that is recruited when GPCR is phosphorylated to stop the signal and end the cycle by causing endocytosis of the GPCR
What does arrestin cause?
endocytosis of the GPCR and the stop of the signal
What most commonly happens to the GPCR after it is endocytosed?
it will dissociate from the ligand in the endosome and return to the plasma membrane after a few minutes
How can a signal be stopped for a longer time?
the GPCR can be degraded in the lysosome
What does RTK stand for?
Receptor Tyrosine Kinases
What are RTKs?
enzyme-coupled receptors
What structure do RTKs have when they are inactive?
Monomers
What structure do RTKs have when they are active?
dimer
What are the 2 ways an RTK can become active?
a joint ligand can bind to the EC side of two inactive monomers and the monomers bind to form a dimer
OR
a single ligand can bind to one inactive monomer and a separate single ligand can bind to another inactive monomer, then the 2 monomers can bind to form a dimer
What side do the ligands bind to RTKs?
the extracellular side
Describe RTK dimerization
A ligand binds to the EC side of an RTK monomer and 2 monomers dimerize
What is the result of RTK dimerization?
The tyrosine kinase domains on the cytosolic sides of each monomer are brought close enough together that the kinase domains can trans-phosphorylate the tyrosine on the opposite monomer
Which amino acid and which enzyme are involved with RTKs?
amino acid = tyrosine
enzyme = kinase
What side of the monomeric RTKs are the tyrosine kinase domains?
cytosolic
What is the function of the kinase domain on RTKs?
When close enough together, the kinase domains will ‘trans-auto-phosphorylate’ a tyrosine on the opposite monomeric RTK
When does trans-auto-phosphorylation of RTK occur?
After dimerization when the tyrosine kinase domains are close together
What is trans-auto-phosphorylated?
a tyrosine amino acid on the opposite monomeric RTK of a dimer
What enzymatic activity does RTK have when it’s active?
kinase activity
T or F: trans-auto-phosphorylation only occurs once per RTK dimer
False
many phosphates can be added on the cytosolic domains
Roughly how many human RTKs exist?
60
What are many RTK ligands involved in? What consequence does this have? Explain
growth
consequence: cancer
they can have mutations such as RTKs in cancer cells being fixed as a dimer and always active –> always promoting growth
What are many RTK ligands referred to as? give examples
growth factors
ex. epidermal growth factor nerve growth factor fibroblast growth factor insulin
T or F: insulin receptor is an RTK
true
What happens after an RTK has been trans-auto-phosphorylated in the RTK pathway?
the phosphorylated kinase domains continue to phosphorylate other cytoplasmic tyrosine until there are a lot of P sites
What is the purpose of creating many P sites in the RTK pathway?
they serve as docking sites for other proteins like kinases, transcription factors, 2nd messengers, etc.
What is the P site?
Phosphorylated tyrosine
What kind of proteins can use the P sites as docking sites?
kinases
transcription factors
2nd messengers
What is the purpose of other proteins (like transcription factors or 2nd messengers) docking to the phosphorylated and dimerized RTK?
they will carry the signal forward
What are two examples of things that the RTK pathway regulates?
transcription
calcium release
Why is it strictly called a Receptor Tyrosine Kinase and not a Tyrosine Kinase Receptor?
tyrosine kinase receptor suggests that it’s a receptor that binds tyrosine kinases (recall that an insulin receptor would bind insulin)
but this is NOT what happens here
a LIGAND binds to the enzyme-coupled receptor (RTK) and the RTK has tyrosine and kinase domains already
Describe phospholipase C
A membrane-bound enzyme that cleaves phospholipids at a specific site when it is activated
How is PLC activated?
by specific GPCRs or RTKs
In a GPCR pathway, how can a PLC function?
as the effector instead of adenylyl cyclase
In an RTK pathway, how can PLC function?
it can dock to one of the phosphorylated tyrosines
What kind of phospholipids will PLC cleave once activated?
specific membrane phospholipids with inositol headgroups called PIP2
What is inositol?
alcohol-sugar
What is the key inner leaflet signalling lipid?
phosphatidylinositol-4,5-biphosphate (aka PIP2)
What does PIP2 stand for?
Phosphatidylinositol-4,5-biphosphate
How many molecules does PLC cleave PIP2 into? what are they?
2 molecules
inositol triphosphate (IP3)
diacylglycerol (DAG)
What does IP3 stand for? How is it formed?
Inositol triphosphate
one of the products of PIP2 being cleaved by PLC
What does DAG stand for? how is it formed?
diacylglycerol
one of the products formed when PIP2 is cleaved by PLC
What are IP3 and DAG?
they are second messengers used in signalling
Where is DAG located when formed?
it is embedded in the membrane
What is the function of DAG?
activates a kinase cascade (Protein Kinase C)
Where is IP3 located once formed?
IP3 diffuses through the cytoplasm
What is the function of IP3?
when it diffuses through the cytoplasm it binds to ligand-gated Ca2+ channels in the sER causing the release of Ca2+ into the cytosol
T or F: PIP2 is made in advance
true
Where is PIP2 located on the membrane?
on the inner leaflet of the membrane
In the GPCR pathway, how is PLC activated?
GPCR is first activated when a ligand binds
then PLC is activated by the G-protein
What activates PLC in the GPCR pathway?
a G-protein
What happens when the PLC is activated?
PIP2 is cleaved into DAG and IP3
DAG activates a kinase cascade
IP3 diffuses through the cytoplasm and binds to ligand-gated Ca2+ channels to release Ca2+
What does IP3 bind to in the cytoplasm?
ligand-gated Ca2+ channels in the sER
What kind of processes occur because of PLC?
Ca2+ regulated processes (ex. exocytosis, cell cycle changes)
T or F: Ca2+ is a second messenger
true
What happens when intracellular Ca2+ is released as a result of PLC?
the activation of Protein Kinase C that is bound to DAG
