Midterm 2 - Notes 4 (Part 3) Flashcards
What does Ras do in its natural form? (2)
- It helps regulate entry into the S phase
2. Monitor GTPase
What is Ras involved with?
Cell cycle control
- entry into the S phase
What does Ras encode?
A small GTP binding protein called GTPase
What is Ras activated by?
GEF
- turns on
GEF
Guanine nucleotide exchange factor
What does GEF lead to?
Activation of downstream signalling components
- eg) MAP Kinase
What is Ras inactivated by?
GAPs
- turns off
GAP
GTPase activating enzyme
- enhance intrinsic GTPase activity
What can happen when Ras is mutated?
It can stay active and create mutations
What activates Ras?
A mitogen
What does Ras initiate?
MAP kinase cascade
MAPK
Mitogen activated protein kinase
What does Ras activates?
MAP kinase kinase kinase
- phosphorylates MAP kinase kinase
- phosphorylates MAP kinase
- phosphorylates target proteins like TF
What is used to help activate proteins?
ATP
Can you phosphorylate more then 1 protein?
Yes
What do you get through a kinase type cascade?
Strong amplification
What does phosphorylation allow?
Cross connection (activation)
What are the 6 steps in Ras activation?
- Ras activated by exchange of GDP for GTP
- Active Ras recruits, binds and activates Raf
- GTP hydrolysis leads to dissociation of Ras from Raf
- Raf activates MEK
- MEK activates MAPK
- Dimeric form of MAPK translocates to the nucleus
- activates many TF
In the S-phase activation pathway what does binding of a mitogen to a cell surface receptor lead to?
Activation of a specific GEF which leads to the activation of Ras
What does activated Ras initiate?
MAPK cascade
What does activated MAPK phosphorylate?
TF
What does Myc activates?
Transcription of delayed response genes
What is an example of a delayed response gene?
G1/S cyclins
What do cyclins bind to?
CDK
CDK
Cyclin dependent kinase
What complex phosphorylates Rb?
G1/CDK
What is p53?
A famous proto oncogene
Where is mutated p53 most commonly found?
Many types of cancer
Is p53 essential for development?
No
- homozygous null mutations (in a mouse) appear normal
What makes amounts of p53 increase?
Stress
What are 2 examples of stresses that can increase amount of p53 and cause damage?
- UV radiation
2. X-rays
What are 4 kinds of signals that come from tumour cells?
- Hyper-proliferation signals
- DNA damage
- Telomere shortening
- Hypoxia
What is a hallmark of tumour formation?
Hypoxia
What is p53’s primary role?
Prevention of cancer
- is a tumour suppressor gene
- its a TF
Where are most cancerous LOF mutations located?
In DNA binding domains
- when mutated it prevents it to bind to its target gene and acts as a regulatory gene
What does DNA damage induce?
Kinase cascade
What happens when p53 is phosphorylated?
It gets activated
What is produced in high levels?
p53
- as long as there is no signal that activates the protein we get binding
What is one target of p53 transcriptional activation?
p21 protein
What does high turn over rate for p53 lead to?
Leads to little p53 present (in a steady state) in the cell because it is degraded very quickly
What does p21 bind to?
G1/S phase activated cyclin dependent kinase
- G1/S-CDK
What happens when p21 bind to G1/S-CDK?
It inactivates it
What does phosphorylation of p53 prevent?
Prevents binding of p53 to Mdm2
What happens when p53 is bound to Mdm2?
It is targeted for degradation
When is p53 a stable protein?
When it is not bound to Mdm2
What happens when Mdm2 is no longer bound to p53?
It is able to proceed with transcription
What can p21 engage in?
Protein protein interaction
- can bind to CDK
- this inactivates it
What happens if DNA is damaged?
It cant enter into the S-phase
