Session 7.2: Signal Transduction 2

Question 1

how do GPCR’s cause a change in cellular activity

Answer 1

1. agonist/ligand bound
2. conformational change
3. activated receptor interacts with G protein
4. G protein (heterotrimeric but beta and gamma stay together and alpha separate). so the receptor interacts with an alpha, beta, gamma subunit with GDP attached
5. this GPCR-G protein interaction activates the G protein by causing GTP to exchange for GDP on alpha subunit
6. now GTP bound to alpha subunit, alpha and beta/gamma interaction weakened and they dissociate so each can go on to cause their own effects by binding with effector proteins

Question 2

where is the alpha, beta, gamma subunit

Answer 2

inner part of plasma membrane as lipid so a stay a part of

Question 3

what are some examples of effector proteins

Answer 3

second messenger-generating enzymes or ion channels

Question 4

how long does activation of GPCR last and what causes this

Answer 4

alpha subunit contains GTPase which can hydrolyse GTP to GDP, then alpha subunit gains affinity for beta-gamma subunit and reform heterotrimer which is inactive

Question 5

what is a key feature of G protiens

Answer 5

diverse - over 1000 possible alpha, beta, gamma protein combinations

Question 6

what governs receptor g protein selection

Answer 6

activated GPCR’s preferentially interact with specific types of G protein where alpha subunit is primarily wanted. the alpha and gamma/beta subunits determine which effector they interactor with…thus signal will activate a specific type of G protein and effectors in the cell so specific cellular response

Question 7

what are the different G protein(GPCR) subtypes

Answer 7

ones that contain:
alpha S subunit - stimulate adenylyl cyclase
alpha I subunit - inhibits adenylyl cyclase
alpha q subunit - stimulate phospholipase C

Question 8

what is the result of adrenaline or noradrenaline binding

Answer 8

adrenaline/noradrenaline binds to beta-adrenoreceptors which are activates and interact with G proteins that contain alpha S and stimulate adenylyl cyclase activity
or can bind and activate an alpha2-adrenoreceptor which interact with g proteins that contain alpha I subunit and will lead to inhibition of adenylyl cyclase
or to alpha 1 adrenoreceptor, interact with g protein that contains alpha q and stimulates phospholipase C activity

Question 9

what is the result of acetylcholine binding

Answer 9

to M2/4 muscarinic receptor, activates, interacts with G proteins that contain alpha I and inhibits adenylyl cyclase
to M1/M3 muscarinic receptor, activates, interacts with G protein that contain alpha Q subunit and stimulates activity of phospholipase C

Question 10

what can interfere with G protein function

Answer 10

toxins - cholera toxin (CTx) and pertussis toxin (PTx)
- toxin complex binds to cell and an enzyme (component of toxin) injected into cells and interfere with G protein and cause disease…(also used to study GPCR-G protein signalling)

Question 11

how does pertussis toxin work

Answer 11

whooping cough
toxin released by bacteria, in airways, interact with epithelial = involuntary and violent coughing
gets into cell, interact with G alpha I proteins and modifies the protein so alpha I proteins roles inhibited. so when activated, interacts with alpha I protein, no longer undergo GTP->GDP exchange so G protein no longer activated =
uncouples Gi-preferring GPCR’s from mediating signal transduction events

Question 12

how does cholera toxin work

Answer 12

produced by cholera, caused dehydration and diarrhoea
enters cell, binds to alpha S subunits and covalently modifies alpha S proteins. so when stimulated protein unable to be inactivated as no GTPase so irreversibly activated as GTP not hydrolysed
= prevents termination of signalling by G preferring GPCRs so long lasting activation of pathways