MCB Lecture 41 Beta-Catenin, Ion Channel receptors, GPCRs

Question 0

What is the importance of the beta catenin pathway?

Answer 0

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

Question 1

What is an example of a pathway that relies on the proteolysis of regulatory proteins?

Answer 1

The Wnt pathway

Question 2

How does the Wnt pathway lead to cancer?

Answer 2

Over expression of the Wnt pathway leads to too much gene expression, and over proliferation of cells

Question 3

Describe how beta catenin levels can rise in the cell cytosol

Answer 3

1. Mutation in APC, beta catenin is not targeted for degredation
2. Constitutively active receptor. The pathway is always on

Question 4

What is the ligand in the Wnt pathway?

Answer 4

Wnt

Question 5

What is the receptor in the Wnt pathway?

Answer 5

Frizzled

Question 6

Describe the signal-transduction of the Wnt pathway

Answer 6

1. Wnt ligand binds to the Frizzled receptor
2. Dishevelled protein is activated
3. The kinases in the beta catenin destruction complex (GSK3B and CK1) are inactivated, so they cannot target beta catenin for degredation
4. Beta catenin levels build up in the cytosol
5. Beta catenin enters the nucleus, kicks off Groucho and binds to coactivators
6. Gene transcription occurs (Wnt genes transcribed)

Question 7

Describe the structure and function of the beta-catenin destruction complex

Answer 7

Axin
APC
GSK3B
CK1

Question 8

What is Groucho, and how is it involved in the Wnt pathway?

Answer 8

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

Question 9

Describe how beta catenin is normally broken down

Answer 9

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

Question 10

Which two proteins phosphorylate beta catenin for targeting by E3 ubiquitin Ligase complex?

Answer 10

GSK3B

CK1

Question 11

Describe how beta catenin mutations affect skin cell differentiation

Answer 11

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

Question 12

Describe how different mutations in beta catenin can affect the cells in the eye

Answer 12

WT: epithelial and fibre cells
B catenin KO: no epithelial cells
APC KO: over proliferation of epithelial cells

Question 13

Describe how beta catenin mutations can affect the Adherens junctions in eye cells

Answer 13

WT: Adherens junctions in the epithelial layer of lens
B Cat KO: no Adherens junctions
APC KO: too many Adherens junctions

Question 14

What is APC, and what do mutations in APC cause?

Answer 14

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

Question 15

Wnt signals regulate … and …

Thus, mutation causes …

Answer 15

Myc and cyclin D

Over proliferation

Question 16

How do colon polyps form?

Answer 16

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

Question 17

Describe the structure and function of ion channel receptors.

Where are they commonly found, and what do they do here?

Answer 17

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

Question 18

Describe the main players in a GPCR signal transduction pathway

Answer 18

Ligand
GPCR
G protein
Target protein

Question 19

Describe the structure of a GPCR

Answer 19

7 pass transmembrane protein

Question 20

Describe the structure of a G protein

Answer 20

Three subunits: alpha beta gamma

Alpha and gamma are membrane bound

Question 21

Describe the differences in G protein suture when it is active and inactive

Answer 21

Active: GTP bound, alpha subunit dissociated
Inactive: GDP bound

Question 22

Describe the signal transduction pathway in GPCRs

Answer 22

1. Ligand binds
2. Conformational change in GPCR
3. G protein is activated: GPCR acts like a GEF: alpha subunit has GDP replaced with GTP
4. Alpha subunit dissociates and moves to a target protein in the membrane
5. Alpha activates the target protein
6. Alpha’s GTPase function is now activated, and GTP is hydrolysed
7. Alpha is inactive, and binds back to beta and gamma subunits

Question 23

How does the alpha subunit of the G protein replace GDP with GTP?

Answer 23

The GPCR acts like a GEF

Question 24

What causes activation of the Target protein?

Answer 24

The association with the activated alpha subunit

Question 25

What causes the hydrolysis of GTP on the alpha subunit?

Answer 25

1. Binding to the target protein

2. RGS

Question 26

What is RGS?

Answer 26

Regulator of G protein Signalling

It activates the GTPase activity of the alpha subunit: GTP is hydrolysed and the alpha subunit is deactivated

Question 27

What is a common target protein?

Answer 27

Adenyl cyclase

Question 28

Describe the pathway that results when adenyl cyclase is activated

Answer 28

1. Adenyl cyclase is activated by alpha subunit of G protein
2. Adenyl cycle catalyses the conversion of ATP to cAMP
3. Much cAMP is produced (amplification)
4. cAMP activated PKA
5. PKA moves to the nucleus and activates the CREB portein
6. CREB binds to CBP
7. Gene transcription is activated

Question 29

What is CREB? How does it function?

Answer 29

It is a protein that is activated by PKA that then goes on to activate gene transcription

Question 30

What causes Albright Hereditary Osteodystrophy?
What is it also known as?

What are the clinical features?

Answer 30

This is a mutation in G proteins.

It causes resistance to parathyroid hormones

Aka Pseudohypoparathyroidism

Obesity
Short stature
Bone malformation: short bones

Question 31

Describe how the beta-gamma subunits are involved in signalling

Answer 31

1. Once the ligand binds to the GPCR, the beta and gamma subunits are also activated.
2. These subunits move to an ion channel in the membrane and cause it to open
3. The K channel opens, and potassium moves down its electrochemical gradient, out of the cell

Question 32

Describe how photo transduction is an example of GPCRs

Answer 32

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)

Question 33

Which molecule is responsible for stem cell differentiation in the skin of mice?

Answer 33

Beta catenin

Question 34

What features are reliant on beta catenin in the lens?

Answer 34

Epithelial cell layer
Adherens junctions

Loss of function: no Adherens, no epithelium
Gain of function: too much epithelium, too much adherens

Question 35

Is APC a tumour supressor or a proto oncogene?

Answer 35

It is a tumour suppressor

Question 36

Describe the important process in bone that relies on G protein coupled receptors

Answer 36

Bone resorption relies on a GPCR and a parathyroid hormone

Mutations in the G protein lead to Pseudohypoparathyroidism / Albright hereditary osteodystrophy