Signal Transduction Flashcards
1
Q
What different changes in cell behaviour can be done due to an extracellular signal?
A
- altered metabolism
- altered gene expression
- altered cell shape or movement
2
Q
What are 4 different types of signalling?
A
- contact dependent signalling
- paracrine
- synaptic
- endocrine
3
Q
What is contact dependent signalling?
A
- the extracellular signal cannot diffuse
- attached to signalling cell
- must be within close proximity
4
Q
What is paracrine signalling?
A
- signals are operating over short ranges
- ligands have structural features that restrict their diffusibility
- important for local modifications (e.g. in wound healing)
5
Q
What is synaptic signalling?
A
- nervous system
- neuronal cell types
- vesicle packets released at synapses
- signalling can only occur within synapses
6
Q
What is endocrine signalling?
A
- specialised cell types that are releasing the signalling molecule into the blood stream
- made available to target cells
- long range signalling
- e.g. beta cells releasing insulin
7
Q
What are some specifications that a signalling machine must meet?
A
- recognise the extracellular signal
- generate an inter-cellular signal
- elicits change in cell behaviour
8
Q
What are some important properties of a signalling machine?
A
- specificity
- sensitivity (affinity for ligand)
- dynamic range (range of concentrations and what responses are given)
- duration (timing of the response)
- processing (timing + shape of the signal. example is a sigmoidal graph, increasing conc will increase response. Or hyperbolic or all-or-none response)
- integration (multiple signals impacting cells)
- feedback and noise (positive and negative feedback, can amplify a forward reaction)
9
Q
What is dynamic range?
A
difference between the smallest and largest usable signal through a transmission or processing chain or storage medium
10
Q
What are the three classes of receptors that respond to signals?
A
- ion-channel coupled receptors
- when ligand binds, conformational change occurs and ion is allowed in
- e.g. GABA-A receptors, ligand is GABA, a inhibitory neurotransmitter - g-protein coupled receptors
- function by switching mechanism that involves GTP and GDP - enzyme-coupled receptors
- when ligand binds, leads to activation of a protein/enzyme catalysis process
11
Q
What are G protein coupled receptors?
A
- largest family of receptors
- conserved structure
- small molecule ligands (acetylcholine)
12
Q
What is the structure of GPCR?
A
- 7 transmembrane helices
- extracellular ligand binding site
- hydrophobic pocket is created by the helices which the ligand can bind
13
Q
What happens when a ligand binds to the G-protein coupled receptor?
A
- ligand binds in hydrophobic pocket
- this elicits a transmembrane conformational change in the conformation of the helices
- intracellular signalling is initiated by sensing the conformational switch
14
Q
How does the GPCR signal via G protein switches?
A
- 3 subunits, alpha, beta and gamma
- GDP is bound to the alpha subunit (inactive state)
- the ligand binding induces G protein binding to receptor
- this induces the exchange of GDP to GTP
- breaks up subunits ti Galpha and Gbetagamma
- the GTP bound form can bind effectors (GTP binds to the alpha subunit)
- GTP is hydrolyses back to GDP
15
Q
What are GPCR’s inhibited by?
A
Arrestins
- proteins which are recruited to the GPC when it is phosphorylated by GRKinase
- arrestin locks the GPCR in its inactive state and degrades signal
16
Q
Name an enzyme coupled receptor.
A
Receptor Tyrosine Kinases (RTKs)
17
Q
What are receptor tyrosine kinases?
A
- an enzyme coupled receptor
- have intrinsic kinase activity
- phosphorylate substrates on tyrosine residues
- ligand binding outside the cell activates the kinase inside the cell
18
Q
What is the structure of the tyrosine kinase domain?
A
- have 2 domains linked together by a flexible region
- N terminal domain contains an ATP binding site
- C terminal domain contains substrate binding site and 2 alpha helices that determine specificity by the exposed amino acids in the helix
- activation loop in the C terminal domain
- contains one or more Tyr molecules
- binds to catalytic cleft
19
Q
What 2 conformations is the activation loop in for tyrosine kinase domain?
A
unphosphorylated form - sits in the substrate binding site
phosphorylated form - the loop flips away from the binding site
20
Q
How do drugs inhibit RTKs?
A
bind to the ATP binding site on the N terminal domain
21
Q
How does the RTK become activated?
A
- receptor dimerisation and inter-molecular phosphorylation
- forced dimerisation activates RTK signalling in the absence of ligand
- ligand acts to dimerise receptors
22
Q
What is achondoplasia?
A
- commonly known as dwarfism
- premature stopping of long bone elongation
- mutations in FGFR3
- causes transmembrane domain to dimerise in the absence of ligand
23
Q
What is crozoun syndrome?
A
- genetic disease resulting in premature fusion of skull bones
- mutations in the extracellular domain of FGFR2 which produces cysteine residues
- leading to covalent dimerisation and activation of signalling in the absence of ligand
24
Q
What happens during ‘dominant negative’ receptor mutants?
A
- a receptor is mutated to lack kinase domain
- unable to signal even though they do bind the ligand as usual
- can block a wild type receptor from signalling due to dimerisation
25
What does activation of kinase lead to in RTK?
- inter receptor phosphorylation of the tyrosine residues seen in C terminal tail of receptor
- auto phosphorylation
- recruitment of substrates
- this causes binding sites for signalling proteins
- activated signalling proteins relay signals downstream
26
How does RTK recruits its substrates?
- by specific recognition of pTYR by SH2 and PTB domains
- these domains have tyrosine phosphate specific binding functions
- binding site for phosphotyrosine and amino acid side chain
27
How does the signal start from when the RTK is activated?
1. The SH2 domain of the adaptor protein (Grb2) is recruited to the activated RTK tails
2. Recognition domain SH3 in Grb2, recruits a protein called Sos (Ras-GEF)
3. When Sos is recruited, it interacts with inactive Ras protein (which is a G-protein) and displaces GDP with GTP and activates Ras
4. Can interact with downstream pathways
28
What downstream pathway is activated by Ras GTPase?
- MAP kinase cascade
- sequence of kinases that phosphorylate each other
- terminal phosphorylase (Erk) phosphorylates and initiates a range of changes of behaviour
29
What is the MAP kinase pathway?
1. Activated Ras protein binds to Raf
2. Raf has one substrate which it phosphorylates- (Mek)
3. Activated Mek has one substrate phosphorylates- (Erk)
4. Erk phosphorylates many other proteins that undergo changes
30
What is the PI3 Kinase pathway?
PI3 Kinase is an enzyme that phosphorylates a specific membrane lipid
- increasing the conc of PI-3,4,5-triphosphate
- that is a docking site for some proteins
- so is recruiting proteins to the membrane hence changing its behaviour
31
What does recruitment of effectors to the membrane due to PI-3,4,5-triphosphate do?
1. recruits 2 protein kinases (PDK1 and Akt) with PH domains (domains that recognise PI-3,4,5-triphosphate)
2. PDK1 phosphorylates Akt
3. Akt activates mTOR (regulator of biosynthesis and energy balance)
4. Akt also phosphorylates a protein called Bad
- inhibits apoptosis in proteins
32
What are cytokine receptors?
- family of structurally related ligands and receptors
- ligand recognition mechanism
- act via receptor dimerisation
- signal via activation of JAK (just another kinase lol) family through phosphorylation of the STAT family
33
Why are cytokines medically important?
```
erythropoetin:
- induces proliferation of erythroblasts
- treatment for anemia
G-CSF:
- induces the proliferation of neutrophils
- treatment with chemo
```
34
What is the structure of the receptor that growth hormones (and cytokines) bind to?
- has cytokine homology domain where 2 modules are at right angle to each other
35
What is the structure of the growth hormone?
- 4 alpha helices
36
What is the activation mechanism for growth hormones to their receptors?
- receptor dimerises through interactions between specific residues in the ligand with residues found in the loops of the 2 modules
- binding of the ligand locks the dimerisation of the receptor
- activates the downwards signalling
37
What happens once the JAK kinase has been activated?
- JAKs have 2 catalytic domains
- the dimerisation results in phosphorylation of the C terminal region of the receptor
- The C terminal region recruits STAT
- STAT has an SH2 domain that binds to the phosphorylated receptor
- STAT then becomes phosphorylated by the JAK Kinase
- Then dissociates from the receptor and forms a dimer through the SH2 domains binding to the phosphorylated sites
- STATs then relocate to the nucleus where is interacts with DNA
