Cell signalling - G protein, Tyr-Kinase, Ras-Erk, AKT/PKB Flashcards
What are the four main types of cellular signalling?
contact dependent
paracrine
autocrine
endocrine
What is contact dependent cellular signalling?
- direct signalling
- signalling cell and target cell must be very close to each other
- ligand on the signalling cell binds to the receptor on the target cell
What is paracrine cellular signalling?
- signalling cell secretes ligands which bind to nearby local targets
- elicits fast responses
- response only lasts a short time = due to degradation or removal of the ligands
What is autocrine cellular signalling?
- signalling cell is the target cell = target site is on the same cell
- cell secretes ligands and the ligand binds to receptors on the same cell
What is endocrine cellular signalling?
- endocrine cells secrete hormones
- hormones secreted enter and travel through the bloodstream
- hormones leave the bloodstream and bind to receptors on distant target cells
- elicit slow but long lasting responses
What are the different types of receptors?
intracellular receptors
- found inside the cytoplasm
transmembrane
- are on/in the cell surface
= ion channels, G protein coupled, tyrosine kinases, proteolysis linked
What are the general rules of signal transduction?
pathways activation is rapid and transient
pathways are multi-component
- intracellular signalling molecules can comprise proteins, small molecules or lipids
target proteins are post-translationally modified and this results in conformational changes.
What is post translational modification? What are the different types?
post translational modifications = after being formed
- changes the activity of proteins that already exist within the cell
- by adding chemical groups which result in conformational changes
- conformational changes allow them to bind to and activate other proteins
types of modifications
- phosphorylation by kinases
- methylation
- acetylation
- lipid conjugation
enzymes which cause these can be inactivated
- failure too inactivate can lead to uncontrolled cell proliferation
How does the G protein coupled receptor work?
G protein coupled receptor
- is a 7 transmembrane receptor = in the cell surface, alpha helices
ligand binds to the G protein coupled receptor causing a conformation change
change activates the protein
protein recruits the heterotrimeric G protein = alpha, beta, gamma
GDP/GTP exchange
- when GDP is bound to the alpha protein = inactive
- when GTP is bound to the alpha protein = active
Active alpha G and GTP subunit disassociate from the beta and gamma dimer
G alpha protein and GTP subunit move downstream while the beta/gamma G protein dimer remains anchored to the membrane
What are the downstream effectors of the G alpha and GTP subunit?
In the cytoplasm:
- glycogen breakdown
- ion channel regulation
- Ca2+ influx/ Muscle contraction = adenyate cyclase and phospholipase C pathway
- cytoskeletal rearrangements
In the nucleus
- transcriptional activation (CREB)
What is the process for G protein coupled receptor activating phospholipase C? What does phospholipase C do?
G alpha and GTP (guanosine triphosphate) subunit bind to the membrane bound phospholipase C beta
- activates phospholipase C beta
Phospholipase C beta catalyses (breaks down) phospholipids within the membrane into DAG and PIP2
- DAG = diacylglycerol
- PIP2 = inositol triphosphate
PIP2 binds to the its inositol triphosphate receptor on the endoplasmic reticulum
- receptor is an intracellular ligand-gated Ca2+ release channel
- ER is an intracellular store of calcium
release of calcium activates
- myosin light chain kinase
- calmodulin dependent kinase
DAG activates protein kinase C
- phosphorylates proteins
- causes smooth muscle contraction
What is the process for G protein coupled receptor acting on adenylate cyclase? What is the result of activating adenylate cyclase?
G alpha and GTP (guanosine triphosphate) subunit bind to the membrane bound adenylate cyclase
- catalyses the conversion of ATP to cAMP
- cAMP acts as a second messenger
cAMP binds to protein kinase A and activates it by causing it to undergo a conformational change
PKA then goes onto to phosphorylate other substrates
How does the Receptor Tyrosine Kinase work?
ligand molecules bind to the extracellular domains of the receptors
binding causes the receptors to dimerise
- come together and associate
- close proximity of the cytoplasmic tails causes the tyrosine kinase activity of the tails to be turned on
receptors then cross phosphorylate each other
- is called autophosphorylation
an effector is recruited
- phosphorylated cytoplasmic tails act as binding sites for the effectors/proteins with a SH2 domain (SH2 groups facilitates binding)
binding of the effector to the cytoplasmic tail causes a conformational change
effector is phosphorylated. phosphorylation causes the release of the effector from the dimer
What are the downstream effectors of receptor tyrosine kinase?
adapter - GEF causes G protein activation = Ras
phospholipase C gamma - activates protein kinase C/ calcium release
phosphatidylinositol 3-kinases - activate protein kinase B = inhibition of apoptosis
What is the process for Receptor tyrosine kinase and ras? What is the result of activating ras?
MAP kinase cascade
ras is a small G protein anchored/associated with the plasma membrane
growth factor binds to the receptor tyrosine kinase. receptors dimerise and autophosphorylate = kinase activated
RKT recruits the adaptor protein (GRB-2) which has SH2 domain
adaptor protein recruits the guanine nucleotide exchange factor (GEF). GEF binds to the adaptor protein via the SH2 domain
GEF exchanges the GDP on the Ras for GTP. this activates Ras
activated Ras triggers a phosphorylation cascade of 3 protein kinases
- each phosphorylates and activates the next = known as MAP (mitogen activated protein kinase cascade)
- Ras phosphorylates Raf. Raf phosphorylates MEK. MEK phosphorylates ERK. ERK phosphorylates Elk
Elk is a transcription factor and the phosphorylated form is able to enter the nucleus
- expresses genes involved in cell proliferation
