MCB Lecture 41 Beta-Catenin, Ion Channel receptors, GPCRs Flashcards
What is the importance of the beta catenin pathway?
Beta catenin is normally degraded and gene expression is turned off.
However, mutations in APC, or binding of Wnt to the receptor deactivates the beta catenin destruction complex. Beta catenin levels build up and gene transcription is turned on
What is an example of a pathway that relies on the proteolysis of regulatory proteins?
The Wnt pathway
How does the Wnt pathway lead to cancer?
Over expression of the Wnt pathway leads to too much gene expression, and over proliferation of cells
Describe how beta catenin levels can rise in the cell cytosol
- Mutation in APC, beta catenin is not targeted for degredation
- Constitutively active receptor. The pathway is always on
What is the ligand in the Wnt pathway?
Wnt
What is the receptor in the Wnt pathway?
Frizzled
Describe the signal-transduction of the Wnt pathway
- Wnt ligand binds to the Frizzled receptor
- Dishevelled protein is activated
- The kinases in the beta catenin destruction complex (GSK3B and CK1) are inactivated, so they cannot target beta catenin for degredation
- Beta catenin levels build up in the cytosol
- Beta catenin enters the nucleus, kicks off Groucho and binds to coactivators
- Gene transcription occurs (Wnt genes transcribed)
Describe the structure and function of the beta-catenin destruction complex
Axin
APC
GSK3B
CK1
What is Groucho, and how is it involved in the Wnt pathway?
It is a protein bound to coactivators on the Wnt genes, preventing transcription
Beta catenin kicks it off so that transcription of the Wnt genes can occur
Describe how beta catenin is normally broken down
It is bound by the Beta catenin destruction complex
It is phosphorylated by GSK3B and CK1
Once phosphorylated, it is targeted by the E3 ubiquitin Ligase complex
Ubiquitin is added
It is target to a proteosome which degrades it
Which two proteins phosphorylate beta catenin for targeting by E3 ubiquitin Ligase complex?
GSK3B
CK1
Describe how beta catenin mutations affect skin cell differentiation
When there are mutations in beta catenin, it cannot enter the nucleus and turn on gene transcription
As a result, In these tissues, there are no stem cells that can produce hair follicles.
The rat is hairless
Describe how different mutations in beta catenin can affect the cells in the eye
WT: epithelial and fibre cells
B catenin KO: no epithelial cells
APC KO: over proliferation of epithelial cells
Describe how beta catenin mutations can affect the Adherens junctions in eye cells
WT: Adherens junctions in the epithelial layer of lens
B Cat KO: no Adherens junctions
APC KO: too many Adherens junctions
What is APC, and what do mutations in APC cause?
It is a scaffold protein in the beta catenin destruction complex
Mutations in it cause over proliferation of cells (cancer), because there is nothing to degrade beta catenin. Levels of beta catenin rise
Wnt signals regulate … and …
Thus, mutation causes …
Myc and cyclin D
Over proliferation
How do colon polyps form?
There are mutations in APC
The beta catenin destruction complex is no longer active, and its levels build up and there is too much transcription of the Wnt genes. There is over proliferation of cells
Describe the structure and function of ion channel receptors.
Where are they commonly found, and what do they do here?
These are proteins in the membrane that form a channel large enough to allow ions to pass through when a ligand binds
They are commonly found in neurons, and they are involved with changing the polarity across membranes
Describe the main players in a GPCR signal transduction pathway
Ligand
GPCR
G protein
Target protein
Describe the structure of a GPCR
7 pass transmembrane protein
Describe the structure of a G protein
Three subunits: alpha beta gamma
Alpha and gamma are membrane bound
Describe the differences in G protein suture when it is active and inactive
Active: GTP bound, alpha subunit dissociated
Inactive: GDP bound
Describe the signal transduction pathway in GPCRs
- Ligand binds
- Conformational change in GPCR
- G protein is activated: GPCR acts like a GEF: alpha subunit has GDP replaced with GTP
- Alpha subunit dissociates and moves to a target protein in the membrane
- Alpha activates the target protein
- Alpha’s GTPase function is now activated, and GTP is hydrolysed
- Alpha is inactive, and binds back to beta and gamma subunits
How does the alpha subunit of the G protein replace GDP with GTP?
The GPCR acts like a GEF
What causes activation of the Target protein?
The association with the activated alpha subunit
What causes the hydrolysis of GTP on the alpha subunit?
- Binding to the target protein
2. RGS
What is RGS?
Regulator of G protein Signalling
It activates the GTPase activity of the alpha subunit: GTP is hydrolysed and the alpha subunit is deactivated
What is a common target protein?
Adenyl cyclase
Describe the pathway that results when adenyl cyclase is activated
- Adenyl cyclase is activated by alpha subunit of G protein
- Adenyl cycle catalyses the conversion of ATP to cAMP
- Much cAMP is produced (amplification)
- cAMP activated PKA
- PKA moves to the nucleus and activates the CREB portein
- CREB binds to CBP
- Gene transcription is activated
What is CREB? How does it function?
It is a protein that is activated by PKA that then goes on to activate gene transcription
What causes Albright Hereditary Osteodystrophy?
What is it also known as?
What are the clinical features?
This is a mutation in G proteins.
It causes resistance to parathyroid hormones
Aka Pseudohypoparathyroidism
Obesity
Short stature
Bone malformation: short bones
Describe how the beta-gamma subunits are involved in signalling
- Once the ligand binds to the GPCR, the beta and gamma subunits are also activated.
- These subunits move to an ion channel in the membrane and cause it to open
- The K channel opens, and potassium moves down its electrochemical gradient, out of the cell
Describe how photo transduction is an example of GPCRs
Rhodopsin and the co formation all change In Retinal is akin to the ligand binding
A G protein is activated (transducin)
A target protein is then activated by the alpha subunits (cGMP phosphodiesterase)
Which molecule is responsible for stem cell differentiation in the skin of mice?
Beta catenin
What features are reliant on beta catenin in the lens?
Epithelial cell layer
Adherens junctions
Loss of function: no Adherens, no epithelium
Gain of function: too much epithelium, too much adherens
Is APC a tumour supressor or a proto oncogene?
It is a tumour suppressor
Describe the important process in bone that relies on G protein coupled receptors
Bone resorption relies on a GPCR and a parathyroid hormone
Mutations in the G protein lead to Pseudohypoparathyroidism / Albright hereditary osteodystrophy
