Lecture 7 - Cross Talk and Compartmentalisation in Signal Transduction Flashcards
Previously we learnt that signalling is compartmentalised between the extracellular space, cytoplasm and nucleus. Further levels of compartmentalisation?
Endocytosis and endosomes.
What triggers the endocytosis of receptors?
Ubiquitination –> ubiquitin is added to lysine residues on the intracellular side of the receptor. Receptor is endocytosed.
Endosomes can trigger the formation of multivescular bodies. How is a multivescular body formed?
Through endosomal membrane invaginations, forming internal vesicles.
How can we differentiate between cell compartments?
The type of phosphoinositide present.
Since different cell compartments contain different phosphoinositides, an issue arises when lipids are transferred between different compartments, e.g. Golgi and ER. How does the cell deal with this problem?
Uses phosphatases and kinases to convert any incorrect phosphoinositides.
How does signalling end? Name the 4 mechanisms.
- Endocytosis.
- Phosphatases.
- Desensitization of GPCR signalling.
- cAMP is degraded by phosphodiesterase.
Mechanism 1: Endocytosis.
- Ligand bound receptor is endocytosed. However, it still signals because the tail is facing into the cytoplasm.
- Receptor enters a multivesicular body to stop the signal.
- Receptor enters internal vesicles = cannot signal.
- Receptors either degraded in lysosomes or is recycled.
Mechanism 2: phosphatases.
Can act at the levels of the target protein or the receptor itself. Many binding domains such as SH2 and PTB rely on phosphorylated binding sites. Therefore, if the phosphate is removed signal transduction is stopped.
Mechanism 3: desensitization of GPCR signalling.
G-protein linked receptor kinase (GRK) can phosphorylate the GPCR receptor, prompting arrestin binding. Arrestin binding prevents trimeric-G protein binding. They also promote endocytosis to ensure the signal is permanently stopped.
Mechanism 4: cAMP is degraded by phosphodiesterase.
Phosphodiesterase can degrade cAMP to regulate signalling.
G proteins are GTPases that can hydrolyse GTP, making the G alpha subunit inactive. They have their own mechanism of regulation. This can be interfered with my two pathogens. Discuss.
Cholera toxin adds ADP-ribose to G alpha (stimulatory) (arginine residue). Prevents hydrolysis of GTP. Stimulatory protein is permanently activated.
Pertussis toxin makes G alpha (inhibitory) incapable of exchanging GDP –> GTP. Blocks the inhibitory pathway.
There is cross talk between signalling pathways. Where does this crosstalk occur?
- At the level of target proteins.
- At the level of the receptors.
- Compartmentalisation and crosstalk.
Give an example of cross talk at the level of target proteins.
Glycogen synthesis and degredation.
Adrenaline is a GPCR ligand - activates PKA via cAMP which promotes glycogen degredation.
PKB s activated by the insulin signalling pathway (RTK), and it inhibits glycogen synthase 3 kinase, which would normally phosphorylate glycogen synthase and promote glycogen synthesis. Since it is inhibited, glycogen synthesis is promoted.
PKA and PKB have opposite effects.
Cross talk at the level of receptors. In the previous lecture we discussed how the PLC signalling pathway provides cross talk between both GPCRs and RTKs. What does this mean?
Through PLC, various ligands can act on the same signalling pathway. GPCRs = environmental signals.
RTKs = growth factor signals.
Compartmentalisation involves the endosome and scaffold proteins. The presence/absence of scaffold proteins can influence which signalling pathways are activated. Explain the function of the scaffold protein APPL.
APPL is a scaffold protein that binds to the endosomal phosphoinositides. PKB and GSK- beta bind and promote cell survival.
If APPL is not present then PKB remains in the cytosol where it instead phosphorylates TSC2 of the mTOR signalling pathway. Leads to growth control.
