Receptor Mechanisms III Flashcards
1
Q
Outline the basic structure of growth factor receptors and what occurs upon activation.
A
- (Usually) consist of a single transmembrane region
- Activation causes further activation of receptor kinases and multiple signalling pathways(varies depending on cellular environment)
2
Q
What roles do enzyme-linked receptors carry out?
A
- Cell growth
- Differentiation
- Division
3
Q
What is the general mechanism of action for enzyme-linked receptors?
A
- Requires 2 identical ligands which bind and cause a conformational change
- Molecules come close together and interact to form a dimer ( DIMERISATION) and as a result the receptors are brought closer to one another
- Allows receptors to modify one another by phosphorylation. Receptor molecule is phosphorylated by enzymes (AUTO-PHOSPHORYLATION)
4
Q
What is the purpose of phosphorylation of the receptors?
A
- Causes conformational change of the receptors and provides a ‘docking site’ where proteins can bind
5
Q
Outline tyrosine kinase activity
A
- Dimerisation brings two receptor molecules together, allowing for the phosphorylation of specific tyrosine residues.
- The phosphotyrosine together with surrounding amino acids are recognised by SH2 domains of other proteins, allowing them to bind and undergo activation.
6
Q
Give another example of a pathway that can occur when tyrosine kinases are activated.
A
- PIP3 kinases can also be activated and phosphorylate PIP2
- PIP3 is formed which combines with and activates proteins such as PDK1 and PKB
- PKB dissociates and causes activation of other proteins
7
Q
Give another example of a pathway activated through the activation of tyrosine kinases
A
- Activation allows formation of regions that can be recognised by the Grb-2 protein
- Grb-2 protein binds to the SH2 domain on the receptor and Grb-2 undergoes conformational change
- Grb-2 interacts with the GEF protein at SH3 domain
- GEF undergoes conformation and facilitates the activation of RAS protein which has a GDP molecule attached
- When activated, RAS exchanges GDP for GTP
- Activated RAS and GTP goes on to activate other intracellular pathways
8
Q
Outline the general structure and function of insulin receptors
A
- Involved in reduction of blood glucose concentration
- It consists of 2 α and 2 β subunits linked by disulphide bridges. It also has tyrosine kinase activity.
- Insulin binding causes autophosphorylation of the 2 β subunits
- Receptor activation in turn causes the activation of IRS proteins
9
Q
Outline what occurs when insulin binds to its receptor
A
- Insulin binding causes conformational change and brings the 2 β subunits close together
- IRS binds via the SH2 domain
- PIP3 kinase activates PDK1 and PKB
- PKB leads to glycogen synthesis and movement of vesicles towards plasma membrane causing increased uptake of glucose
10
Q
How are receptor kinases inactivated?
A
- Dephosphorylation of the receptors
- Phosphatases are activated - therefore further protein activation is inhibited
- Receptor internalisation - the receptor may be completely removed from membrane or broken down
11
Q
Describe the Smad-dependent signalling pathway.
A
- TGF-β binds to a Type II TGF-β receptor.
- Binding allows the association of the Type I TGF-β receptor with the Type II TGF-β receptor
- The serine/threonine kinase domain on the Type II receptor phosphorylates the Type I TGF-β receptor.
- The Type I receptor activates SMAD 2 or SMAD 3 molecules. This allows them to interact with a subsequent SMAD molecule called SMAD 4.
- SMAD 4 will then be translocated into the nucleus, bind with gene regulatory proteins, act on the TGF-β target gene and influence gene expression.
12
Q
Describe the JAK/STAT signalling pathway.
A
- This pathway is activated by growth hormone.
- When the receptors bind, they dimerise. They have JAKs which cross-phosphorylate and phosphorylate the receptors.
- These phosphorylated tyrosines act as docking sites for STATs. When the STATs dock, the JAKs phosphorylate them. Then, the STATs dissociate from the receptor and dimerise
- Dimerised STAT then enter the nucleus, bind with gene regulatory proteins, act on the target gene and increase transcription.
13
Q
Describe the Ras molecule.
A
- Ras is one of a number of small G-proteins. It’s related to the Gα subunit of cAMP.
- Active when bound to GTP, not active when bound to GDP
- It’s involved in growth, differentiation cell motility
- It has GTPase activity, and involved in slow de-phosphorylation helped by GTPase-activating proteins (GAP).
14
Q
How is Ras activated?
A
- When it is GDP-bound, the Ras molecule is inactivated. With the help of GEF (Guanine-nucleotide Exchange Factor), the GDP is replaced for a GTP molecule. With Raf binding to the Ras molecule, it is now active.
15
Q
How is Ras deactivated?
A
- When it needs to be inactivated, GAP (GTPase Activating Protein) assists in the dephosphorylation of GTP to GDP. This means that the RAF now dissociates from the Ras molecule, and it is now rendered inactive.
