RTKs and ligand gated ion channels

Question 1

Receptor-tyrosine kinases

Definition

Answer 1

Enzyme-coupled receptors with a cytosolic domain which has intrinsic tyrosine kinase activity.
The domain phosphorylates tyrosine amino acid residues.
The receptor is activated by ligand binding to the extracelular domain of the receptor.

Question 2

RTK activation

(5)

Answer 2

• Usually exist as inactive monomers.
• Binding of a ligand brings two monomers closer together, resulting in dimerization.
• The tyrosine phosphate domains on each monomer will cross autophosphorylate each other, and become active.
• The activated domains will then phosphorylate other tyrosine residues outside of the kinase domain.
• These phosphotyrosines will act as docking sites for adaptor or signalling proteins which will propagate the signal to the rest of the cell.

Question 3

HER2

Answer 3

• An RTK
• Human epidermal growth factor.
• No high affinity ligand of its own, so normally forms heterodimers with other receptors in the family.
  HER2 overexpression causes HER2 to form heterodimers with itself, this can cause downstream Ras binding to occur regardless of ligand binding.

Question 4

RAS

Answer 4

• Ras is a monomeric g protein with intrinsic GTPase activity.
• It Can hydrolyse GTP to GDP
• In its GDP bound states, it is inactive. In its GTP bound states it is active.
• Ras-GEF promotes Ras to release GDP in exchange for GTP, activating it.
• Ras-GAP accelerates Ras to hydrolyse GTP.

Question 5

Phosphorylation

(5)

Answer 5

• Active form is the phosphorylated protein.
• Inactive form is the phosphorylated version.
• Protein kinases phosphorylate and activate proteins.
• Protein phosphatases dephosphorylate proteins.
• This is only usually the case, sometimes activation and deactivation are the other way around.

Question 6

Main types of RTK signalling pathways

(3)

Answer 6

1. Ras MAP activation
2. PI3K-AKT
3. Phospholipase C

Question 7

Ras MAP activation

(7)

Answer 7

1. Adaptor protein (GRB2) docks onto phosphorylated tyrosine on receptor tyrosine kinase
2. GRB2 is associated with a regulatory protein called Ras-GEF (or Sos)
3. Sos promotes inactive Ras to release GDP and bind GTP.
4. GTP bound RAS can activate MAP kinase kinase kinase (Raf).
5. Raf phosphorylates and activates MAP kinase kinase (Mek) using ATP.
6. Mek phosphorylates and activates MAP kinase (Erk) using ATP.
7. Erk then can phosphorylate different proteins and transcription factors.

In this chain, amplification is occurring as each kinase is able to activate multiple kinases.

Question 8

MAP

Stands for

Answer 8

Mitogen activated protein

Question 9

PI3K activation

(6)

Answer 9

• Activated PI3 kinase docks on to phosphotyrosines of HER3 and is activated.
• Uses PIP2 as a substrate to generate PIP3.
• Protein kinase 1 and Akt use PIP3 as a docking site through these PH domains.
• This allows these proteins to be brought together at the plasma membrane.
• Protein kinase 1 phosphorylates and activates Akt. Other protein kinases also do this.
• Phosphorylated Akt dissociates.

Question 10

PIP

Answer 10

Phosphatidylinositol triphosphate
PIP 1 and 2 are membrane bound

Question 11

Pleckstrin homology domains

Answer 11

proteins region that allows high affinity binding to PIPs.

Question 12

Genes in both pathways are often implicated in cancer and are called…

Answer 12

Proto-oncogenes

Question 13

AKT

Answer 13

• Active Akt is a serine/threonine kinase.
• Akt phosphorylates Bad which holds Bcl2 in an inactive state. When phosphorylation occurs, Bad releases Bcl2. Bcl2 promotes cell survival by inhibits apoptosis.
• Akt activation results in the downstream activation of mTOR which inhibits protein degradation and stimulates protein synthesis.

AKT is important for cell survival and cell growth.

Question 14

Phospholipase C pathway

Answer 14

• Phospholipase C-γ directly binds to a phosphotyrosines of HER3 and is activated through phosphorylation.
• Phospholipase C-γ then uses PIP2 as a substrate to generate DAG (diacyl glycerol) and IP3 (inositol triphosphate).
• IP3 is released into the cytosol, where it binds and opens calcium channneles in the endoplasmic reticulum.
• DAG Recruits and activates protein kinase C (PKC).
• Calcium in the cytosol will bind to PKC allowing the phosphorylation of target proteins to have their effects.

Question 15

Ion channels

(4)

Answer 15

• They are membrane transporters and are channel proteins.
• allow for the movement of ions across a membrane.
• They are selective.
• Most ion channels are gated,

Question 16

Mechanically gated ion channels

Answer 16

The channel is mechanically pulled open by a physical force, allowing ions to pass through the channel.

Question 17

Ligand-gated ion channels

Answer 17

The opening of the channel relies on a small molecule/ligand to bind intracellularly or extracellularly to the channel.

Question 18

Voltage-gated ion channels

Answer 18

The opening of the channel is controlled by the membrane potential.

Question 19

Process of action potential travelling from one nerve cell to another

(8)

Answer 19

1. Voltage gated ion channels generate and carry the action potential along the axon and into the presynaptic nerve terminal.
2. Once the action potential reaches the terminal, voltage gated calcium channels open and there is an influx of calcium.
3. This triggers the vesicles to bind to the presynaptic membrane and to release their neurotransmitters into the synaptic cleft.
4. The neurotransmitters diffuse across the cleft to the postsynaptic membrane.
5. They bind to their receptors – transmitter gated ion channel.
6. The binding of the ligand will open the channel and allow the influx of ions.
7. This influx of ions will change the membrane potential of the postsynaptic membrane.
8. This membrane potential can cause an action potential if the threshold is reached.

Question 20

Voltage gated ion channels

neuronal signalling

Answer 20

Generate and carry the action potential along the axon to the presynaptic terminal

Question 21

Ligand-gated ion channels

neuronal signalling

Answer 21

Resposnible for the rapid transmission of signals across synapses in the nervous system.

Question 22

Glycine transmitter-gated ion channel.

(5)

Answer 22

• mediates fast inhibitory nerve signalling in the central nervous system
• motor and sensory information
• Once glycine is bound to the channel, the channel will open.
• Once the channel is open, it allows the influx of chloride ions into the cell, reducing the membrane potential (hyperpolarising), making it harder for the threshold value to be reached.
• It has an inhibitory effect on action potential generation in the spinal cord and brain stem.