Week 11: Signalling, Cross Talk And Cancer Flashcards
What are receptor protein tyrosine kinase (RTK)?
Integral membrane proteins
Single transmembrane helix extracellular ligand binding domain
Cytoplasmic protein trypsine kinase
What are RTK activated directly by?
Epidermal growth factor (EGG)
Platelet derived growth factor (PDGF)
Insulin
What is the crucial step in RTK?
Dimerisation of the receptor
Step 1 - ligand mediated dimerisation
Inactivated: the tyrosine kinase receptors will exist as inactive monomers where a membrane is fluid
Some RTK have two binding sites for one ligand molecule
Bivalent ligand molecules are required to bind to inactive monomers of tyrosine
What are examples of ligand of TRK?
Insulin
Growth factors
Epidermal growth factor (EGF)
Platelet derives growth factor (PDGF)
What are Ligands for RTK?
Hormone
Specifically peptides
Large molecules
Where do hormonal messages circulate?
In bloodstream
Control proliferation of cells
Step 2 - ligand mediated dimerisation
Dimerisation of the receptor
Doesn’t fully activate receptor molecules
Need phosphorylation
Step 3: ligand mediated dimerisation
Following dimerisation, phosphorylation occurs
Activation of kinase activity causes kinase to catalyse the addition of phosphate groups to tyrosine on the cytoplasmic domain of receptor subunits
What is the consequence of bringing two kinase domains in close contact?
Allows for trans autophosphorylation
Protein kinase activity of one receptor of diner phosphorylates the tyrosine residues in the cytoplasmic domain of the other receptor of diner
What does kinase activity of receptor cause?
Phosphorylation
Step 4 - ligand mediated dimerisation
Newly formed phosphotyrosine kinase
recognised by proteins within the cell
Each protein binds to a specific tyrosine kinase
Inside cell there is inactive relay proteins which are regulated by activated RTK
RTK changes shape and activates relay protein
Each activated protein triggers transduction pathway leading to cellular response
What do residues of receptor serve as?
Binding sites for either SH2 or PTB domains
Receptor mediated dimerisation
Ligand is monovalent
Separate ligand molecules bind to each of the inactivated monomer
Binding of each ligand induces a conformational change in the extracellular domain of receptor (creates dimerisation interface)
The ligand bound monomer interact through this interface and become active dimer
What is present in the cytoplasm?
Signalling proteins with SH2 domain and PTB domains
What does receptor activation result in?
Formation of signalling complexes in which SH2/PTB containing signalling proteins Bind to specific autophosphorylation sites on the receptor
What do adaptor proteins contain?
SH2 domain
Additional protein-protein interaction domain
E.g. GRB2 contains one SH2 and two SH3 domains
What do SH3 domain of GRB2 bind to?
sos and GAB
What does SH2 domain bind to?
Phosphorylated residues of receptor tyrosine kinase within a tyr-x-asn motif a
What does tyrosine phosphorylation of tyr-x-asn motif on RTK result in?
Translocation of GRB2-sos or grb2-gab from cytosol to receptor which is present at the plasma membrane
What does PI3K (phosphoinositide-3-kinase) phosphorylate?
Phosphatidylinositol
How can phosphorylating phosphatidylinositol be regulated?
Activating the phosphorylation of PI through another hormone signal
What does phospholipase enzyme cleave?
Phosphorylated IP2
What are the two ways in which to activate PLC?
GPCR action
Activated by RTK
Ligand
Pathway that activates RAS
What is RAS?
G protein
Monomeric GTPase
RAS
Peripheral membrane protein
Covalently bound hydrocarbon tail and an acyl group which keeps fail at surface of membrane
Active when it has GTP bound
GDP bound - inactive RAS
What is the mode of activation of RAS?
In active form it will drift around on the inner surface of membrane with a GDP in its jaw so when it encounters - bind and undergo conformational change
GDP will dissociate and GTP will bind
Activated RAS protein
What does RAS hydrolyse?
GTP —> GDP
How does desensitisation occur in RTK?
Internalisation of the receptor
What happens after RTK are activated and dimerized?
RTK become internalised into cell via endocytosis
What does it mean when a cell has fewer RTK?
Less sensitive to hormone
What is Grb2-Sos?
An adaptor complex that binds to phosphorylated RTK
Define convergence
Different signals, same response
What is convergence?
Different signals perceived by different receptors but same response at end
Define cross-talk?
One signal transduction pathway influences the operation of a second signal transduction pathway
What is PLC beta activated by?
GPCR
What is PLC gamma activated by?
TRK
What does phosphatidylinositol molecule in membrane release?
diacylglycerol and IP3
What does IP3 cause the release of?
Ca2+ in RER
Ca2+ released into cytoplasm can bind to other proteins and activate them
