Flashcards in Signalling Mechanisms in Growth and Division Deck (41)
What transcription factor is stimulated by growth factor signalling and is vital to starting the cell cycle?
Describe what happens to tyrosine kinase receptors when growthfactors bind to them.
Tyrosine kinase receptors are usually present on membranes as inactive monomers
Most growth factors are dimers, so when they bind they bring tyrosine kinase receptors close together
This allows the tyrosine kinase receptors to cross-phosphorylate each other (using the gamma phosphate from ATP to phosphorylate tyrosine residues in proteins)
The phosphorylated domains on the tyrosine kinase receptors act as docking sites for adaptor proteins
Give an example of an anti-cancer drug that targets tyrosine kinase receptors.
Herceptin – inhibits the Her2 tyrosine kinase receptor (important in many tumours e.g. breast)
Name an important adaptor protein.
Describe the structure of this protein.
It is modular
It has an SH2 domain, which binds to the docking sites (phosphorylated tyrosine residues on the tyrosine kinase receptors)
It has two SH3 domains, which bind to proline-rich regions of proteins
Describe how receptor protein tyrosine kinases can signal to Ras.
Grb2 is bound to an exchange factor called Sos
When the tyrosine kinase receptors become active and the dockingsites become available, Grb2 binds to the docking site and it is also attached to Sos
This brings Sos close enough to the cell membrane and Ras, to allow it to exchange the GDP on Ras for GTP
GTP bound Ras is active
What must the Ras protein be bound to for it to work?
It must be bound to the plasma membrane
NOTE: interference with the membrane binding of Ras can make a good anti-cancer drug
How is Ras turned off?
Ras has intrinsic GTP hydrolysis capability
This GTPase activity is stimulated by GTPase-activating proteins (GAPs)
Broadly speaking, how might Ras signalling be different in cancer?
Ras could be permanently switched on (in the GTP bound form), thus it constantly signals cell division
Describe two mutations that lead to an increase in the amount of active Ras.
V21Ras – glycine is replaced by valine, which means that a simple hydrogen side chain is replaced by a hydrophobic sidechain. This hydrophobic side chain doesn’t allow GAPs to bind to Ras, thus preventing inactivation of Ras.
L61Ras – glutamine is replaced by leucine (in position 61), which means that an amine side chain is replaced by a hydrophobic side chain. This inhibits the GTPase activity of Ras so Ras remains in the active, GTP bound form.
What cascade does Ras activate?
ERK cascade (Extracellular signal-regulated kinase cascade)
What is the family that this cascade belongs to called?
MAPK cascade (Mitogen-activated protein kinase cascade)
What are the three kinases involved in the ERK cascade?
What does the last kinase in the cascade phosphorylate?
It phosphorylates gene regulatory proteins (transcription factors), which go on to regulate the expression of genes involved in the cell cycle
They also phosphorylate other proteins and change their activity
What important gene is turned on by the kinase cascade?
What type of kinase are cyclin-dependent kinases (Cdks)?
What conditions do Cdks require to become activated?
Binding to cyclin
Phosphorylation (activating phosphorylation and removal of inhibitory phosphorylation)
(+ degradation of Cdk inhibitors)
What does the mitosis-promoting factor (MPF) consist of?
Cdk1 + cyclin B
What are the requirements, in terms of phosphorylation, for MPF to become active?
Activating phosphorylation by CAK (Cdk activating kinase)
Removal of the inhibitory phosphorylation (that was placed by Wee1)by Cdc25
What activates MPF at the end of interphase?
Removal of the inhibitory phosphorylation by Cdc25
Describe the positive feedback loop that is formed by MPF activation.
Removal of the inhibitory phosphorylation by Cdc25 produces active MPF, which then phosphorylates Cdc25 and increases its activity meaning that more MPF can be activated
How does MPF put mitosis on hold before progressing to the next stage?
At the end of metaphase, it phosphorylates a number of key proteins and inhibits their action (thus putting mitosis on hold) until it is signalled to proceed. Then the cyclin B is degraded, Cdk is inactivated and the substrates become dephosphorylated and hence become active.
Which Cdk/cyclin is required for G1/S phase?
Which Cdk/cyclin is required for S phase?
How can the same Cdk be used for two different stages?
Cyclin binding alters the substrate specificity of Cdk
Also, different substrates are available at different stages of the cell cycle
What is one of the most important transcription targets of c-Myc?
What is the first Cdk/cyclin complex that is formed when a cell goes from G0 to G1?
This Cdk/cyclin complex then stimulates the expression of the next cyclin in the cell cycle. What properties does this system give to the cell cycle?
This gives the cell cycle direction and timing (because the Cdk-cyclin complexes must reach a certain concentration before they can triggerthe next stage of the cycle)
Give an example of a phosphorylation target of MPF that allows the cell cycle to progress.
Phosphorylation of nuclear lamins allows breakdown of the nuclear envelope