L4 - Ras signalling Flashcards
What is Ras?
Ras is a GTPase - quite a bad one
What is the difference between c-Ras and H-Ras?
H-Ras binds GTP but cannot hydrolyse it
What is a GAP?
GTPase activating protein. Increases the activity of Ras
What is a GEF?
A guanine nucleotide exchange factors. Facilitate the dissociation of GDP from Ras
What is meant by Ras is a 2 state system?
GTP bound Ras is ‘on state’. It is a timebomb as Ras is a GTPase and will eventually cause the hydrolysis of GTP to GDP. In presence of GEF, the dissociation of GDP happens quicker, allowing another GTP to bind and activate Ras. GAP proteins increase the GTPase activity of Ras, facilitating a faster hydrolysis, turning Ras off
What happens when Ras is oncogenic?
3 pathways are switched on - cell growth, gene expression and cell morphology and movement
This causes a cell phenotype with the hallmarks of cancer.
How was Ras identified as a GTP binding protein
Ras was inc with RA guanine nucleotides and the levels of RA could detect binding. In the displacement experiments Ras was incubated with RA guan and other substrates. Both GTP and GDP decreased the amount of RA suggesting that they bind to Ras
What happens in the mutant form of Ras?
In normal Ras - levels of GDP appear after 60 minutes of inc with GTP - RAS IS A GTPase
In mutant Ras - no GDP is present but GTP can bind Ras therefore Ras has lost the ability to function as a GTPase
How were GAPs identified?
Inc Ras, GTP and GDP with different proteins to study their effects. MCF7 increased the amount of GDP present suggesting that it increases the GTPase function of Ras
How were GEFs identified and what protein was found to be a GEF?
Brain cytosol was found to have a concentration dependent activity that facilitates the dissociation of GDP from Ras. Cdc25 was shown to have a homology to this GEF
How does Ras fit into the RTK signalling pathway?
GF binds to RTK activating it. The SH2 domain of GRB2 binds the phosphorylated RTK. The SH3 domain of the GRB2 protein can then bind to a GEF, which facilitates the dissociation of GDP from Ras - allowing GTP to bind and activate Ras
