Cell Signalling Flashcards
Describe communicaion
a process by which information is exchanged between individuals through a common system
Describe a simple signalling pathway
Signal: extracellular activator of the pathway
Reception: detection of signal pathways
Response: change in cellular process
How can you alter the activity of a signalling pathway?
- changing the level of the protein
- changing the activity of a fixed amount of protein via a conformational change or a covalent modification
What are the different cell signalling methods from local to distant range
Gap junction, Contact-dependent, Paracrine, Synaptic, Endocrine
What is a gap junction?
an intracellular gap that allows small signalling molecules to pass directly from cell to cell
What molecules can pass through gap junctions
Ions: Na+, Ca+, K+
Metabolites: sugars, amino acids, nucleotides, AT, cAMP, InsP3
Describe contact-dependent signalling
- not secreted
- signalling molecule on surface interacts directly w receptor on recipient cell
- important for immune signalling and during development
Describe paracrine signaliing
- use of local mediators that act on different cell types in close proximity
- important during inflammation and development
Describe autocrine signaling
- it’s self signalling i.e the cell is able to bind the signal it excretes
- other cells of the same type can bind the signal
- encourages to grow and proliferate
What are the properties of a signal that binds to intracellular receptors
small, hydrophobic i.e steroid hormones and NO gas
What are the properties of a signal that binds to extracellular receptors
hydrophilic as they can’t cross the cell-surface receptors i.e cytokines and neurotransmitters
What are the 3 types of cell surface receptors
- ion channel coupled receptors
- G-protein coupled receptors
- enzyme coupled receptors
What do ion channel coupled receptors do
they convert chemical signals to electrical signals in the nerve
How do ion channel coupled receptors work
the binding of a ligand induces a conformational change in the receptor
What is an example of ion channel coupled receptors
Nicotinic acetylcholine receptors on skeletal muscles
How do nicotinic receptors work
when ach binds to the subunits causing a conformational change allowing Ca2+ into the cell
This causes a depolarisation therefore there is contraction
What are nicoinic receptors an example of
ionotropic receptors and fast receptors
What causes Myasthenia Gravis
Auto-antibodies block nicotinic receptors causing muscle weakness
What are Trimeric G proteins
3 subunits; alpha, beta and gamma
they’re transducers linked to G-protein linked receptors
convert one signal from one form to another
What are monomeric G proteins
single subunits activated when attached to GTP
transduce signals from enzyme linked receptors
Describe the alpha-subunit within the trimeric G protein
it has intrinsic GTPase activity and binds GDP in its resting state
it conatins2 subdomains; RAS and AH
Describe the function of the Ras and AH domains
RAS: one face of the nucleotide-binding pocket
AH: clamps the nucleotide in place
Describe the steps leading up to the dissociation of the beta/gamma subunits and the activation of the alpha subunit
1- signal induces conformational change in receptor and alpha-subunit
2- alpha-subunit releases GDP and binds GTP activating it
3- the beta/gamma subunits dissociate
How does the dissociation of an activated alpha subunit allow for amplification
the receptor is still activated and can bind to a new g protein
1 receptor can activate many g proteins
How do G proteins get ready to interact w another GPCR
after the activated G-proteins transduce the signal they turn themselves off using their intrinsic GTPase
the inactive alpha subunit reassembles w the beta/gamma subunit
What are the 2 main secondary messengers produced
Adenylyl Cyclase -> Cyclic AMP (cAMP)
Phospholipase C -> Inositol 1,4,5 triphosphate (IP3) AND diacylglycerol (DAG)
What are GEFS
Guanine nucleotide exchange factors; activate GTP-binding proteins and promote the release of bound GDP
What are GAPs
regulator protein that increases the rate of hydrolysis of bound GTP
How does cAMP pass information down the signalling pathway
it activates a cAMP dependent protein kinase A (PKA)
causes a conformational change and releases 2 catalytic subunits
What is the structure of cAMP dependent protein kinase A (PKA)
it has 4 subunits; 2 regulatory and 2 catalytic that is inactive while bound
What is the function of phosphatases
to remove the phosphate from phosphorylated proteins
What does phosphorylation of proteins do to their function
switch them on and off
What is a kinase cascade
a protein kinase activated by phosphorylation which then phosphorylates the next protein
What is the importance of epinephrin
it activates glycogen phosphorylase which is inactive without phosphate
it then hydrolyses glycogen to glucose 1-phosphate
Describe the epinephrine signalling pathway
1- epinephrine activates the GPCR, activating the G-protein
2- adenylyl cyclase converts ATP -> cAMP which activates PKA
3- a phosphorylation cascade phosphorylates glycogen phosphorylase
How is IP3 and DAG produced
the activated G-protein activates phospholipase C
phospholipase C then cleaves phosphoinositol 4,5 -bisphosphate ( PIP2) to produce IP3 and DAG
What does IP3 casue?
the release of Ca2+ from the endoplasmic reticulum
What do Ca2+ and DAG do
they activate protein kinase C (PKC)
Why is Ca2+used as a secondary messenger
the cytosolic Ca2+ are relatively low compared to extracellular levels
therefore small changes are easily detected
What mediates the effects of Ca2+
Calmodulin
How does calmodulin function
the molecule can bind 4 Ca2+ ions
this results in a conformational change allowing the calmodulin/Ca2+ complex to wrap around the target protein
this activates the protein
Describe the structure of enzyme-linked receptors
single span transmembrane protein with an intracellular and extracellular domain
the cytosolic has intrinsic enzymatic activity or is associated w an enzyme
What is a receptor tyrosine kinase (RTK)
the most common type of enzyme liked receptors
they have intrinsic kinase activity
Describe what happens when a ligand binds to an enzyme linked receptor
the binding causes cross-linking of 2 receptor chains
this oligomerisation allows cross-phosphorylation (autophosphorylation) i.e phosphorylate each-other
What is autophosphorylation
when the domain can phosphorylate itself
What is cross-phosphorylation
when each domain can phosphorylate another domain
What is RAS
a small monomeric G protein
main signal transducer for growth factors
Describe how RAS is activated
Its inactive when GDP is bound, this is released by GEF allowing GTP to bind
the GTP is the hydrolysed by GAP and it’s weak intrinsic GTPase activity
Why does RAS require a linker
Because it’s not directly linked to the receptor
What benefits do phosphorylated tyrosine residues have
they provide docking sites for other signalling proteins on the receptor
What binds RAS and GEF to the RTK
mediated by an adaptor protein Grb-2
What is a MAP
mitogen activator protein
Describe the MAP kinase pathway
RAS activates a downstream phosphorylation cascade which can change protein activity or gene expression
Describe the steps in the MAP kinase pathway
1- RAS activates MAPKKK (Raf)
2- MAPKKK activates MAPKK (Mek)
3- MAPKK activates MAPK (Erk)
4- MAPK activates MAPs by phosphorylating the MAP protein
How can a mutation in RAS lead to cancer
RAS is a proto-oncogene
the mutation usually in GTP hydrolysis activity; the GTP stays bound longer and the cycle is continuous
Can cause cell proliferation even in the absence of growth factors
What is an inhibitor
a molecule that can target specific sites in pathway to inactivate it
usually come as monoclonalantibodies
How can different cells respond differently to the same signal
- using different receptors
- activating different intracellular machinery
What are the receptors classes ad based on their speed
Ion channel linked receptor; ionotropic/ fast
GPCR: metabotropic/ slow
Can signalling pathways interact
yes- different responses need different combinations of signals
What is cell crosstalk
when there is an overlap between different signalling pathways i.e secondary messengers shared and signalling proteins
What are the pro vs con of cell cross-talk
pro: allows the fine tuning of a response
con: runs the risk of producing the wrong responses
What are the effects of signals combining
- they can activate a protein
- they can alter the activity of signalling protein
What are the different coordination’s of signals possible
Additive; 1+1+2
Synergism; 1+1 >2 i.e increased effect
Antagonism; 1+1< 2 i.e blocked effect
What are the 3 types of signalling complexes
1- stable; components of the signalling pathway are linked by a scaffold protein
2- transient; complex assembles after receptor is activated
3- transient; modification of plasma phospholipid molecules
What is a scaffold protein
molecule that brings together interacting proteins into signalling complexes
Describe how a signal is removed
1- degradation
2- recycling
3- sequestration
What affects the duration of a response
the rate at which the signal is removed
What are the ways of removing a receptor
1- receptor sequestration; it’s temporary as the receptor is held in an endosome
2- receptor down regulation; permanent as the receptor is degraded by a lysosome
