Flashcards in MR 7 Signal Transduction Deck (29):
What does G-protein stand for?
guanine nucleotide binding protein
Of how many subunits are g-proteins made up?
They are heterotrimeric, being made up of 3 subunits: alpha, beta and gamma. The beta and gamma sub units bind tightly to each other and function as a single unit
Which sub unit binds to guanine?
The alpha sub unit has a nucleotide-binding site, which binds GTP and slowly hydrolyses it to GDP (GTPase activity)
Under basal conditions where is the G protein and in what form?
The inner face of the plasma membrane, in its heterotrimeric form with GDP bound to the alpha sub unit
What happens when an agonist binds to the receptor?
The activated receptor has a high affinity for the GDP bound G protein and a protein-protein interaction occurs causing GDP to be released and GTP bound in its place (receptor acts as guanine nucleotide exchange factor GEF). Once GTP has bound affinity of receptor for both alpha-GTP and beta-gamma subunits is decreased and they release and split apart being able now to interact with effectors
What terminates G-protein signalling?
The GTPase on the alpha sub unit hydrolyses the GTP to GDP and then the alpha-GDP sub unit and the beta-gamma sub unit reform an inactive heterotrimeric complex
If we think of G proteins as on/off switches and timers what represents each?
On switch is receptor facilitated GDP->GTP exchange
Timer/off switch is length of time for GTP hydrolysis
What does Gs(stimulatory) do and what signal and receptor does it respond to?
Stimulates adenylyl cyclase to make cAMP e.g in glycogenolysis and lipolysis
What does Gi(inhibitory) do and what signal and receptor does it respond to?
Inhibits adenylyl cyclase so less cAMP is made (also stimulates K+channel) e.g slowing of cardiac pacemaker
M2 muscarinic receptor
What does Gq do and what signal and receptor does it respond to?
Stimulates phospholipase C in smooth muscle contraction
What does Gt (transducin) do and what signal and receptor does it respond to?
Stimulates Cyclic GMP phosphodiesterase giving visual excitation.
How many alpha beta gamma sub unit combinations are there?
What advantage does the high g protein and receptor diversity give?
Extracellular signal working via specific receptor activates only a signle or small sub-population of g-proteins and effectors in cell to give a specific response
How collectively do the cholera and pertussis toxins affect G-protein function?
They ADP-ribosylate specific g proteins
What G protein does CTx affect and what does it do?
Affects Gs-alpha, eliminating GTPase activity meaning Gs-alpha irreversibly activated
What G protein does PTx affect and what does it do?
Affects Gi-alpha intefering with the GDP/GTP exchange so Gi-alpha becomes irreversibly inactivated
What is retinitis pigmentosa caused by?
loss of function (or gain of function) mutation to rhodopsin
What is nephrogenic diabetes insipidus caused by?
loss of function mutation to V2 vasopressin receptor
What is familial male precocious puberty caused by?
gain of function mutation to luteinizing hormone receptor
Which g proteins is adenylyl cyclase affected by/
Gs and Gi
What does adenylyl cyclase do?
hydrolyses cellular ATP to generate cAMP which interacts with cyclic AMP-dependent protein kinase (PKA) which phosphorylates other proteins within cell to affect activity
What effects can increasing cAMP have?
increased glycogenolysis and gluconeogenesis in the liver, increased lipolysis in adipose tissue, relaxation of smooth muscle, positive inotropic and chronotropic effects in the heart
What does phospholipase c do and what does its product do?
Hydrolyses membrane phospholipid PIP2 to IP3 when activated by Gq
IP3 interacts with receptors on ER to allow Ca2+ release into the cytoplasm from the ER
What causes the receptor desensitisation phenomenon?
Whilst activated receptors are susceptible to a varitey of protein kinases which phosphorylate the receptor and prevent it activating further g-proteins
What factors deactivate g protein coupled receptor pathways?
Once receptor has productively interacted with a g-protein agonist binding is weakened and likely to dissociate
Receptor desensitisation phenomenon
active lifetime of alpha-GTP limited by cellular factors stimulating the GTPase activity of subunit
Basal state is favoured and cells contain high activity enzymes that break down second messengers
Enzymatic cascades downstream to second messenger activation act to oppose effect
How does Gi effect chronotropy in the heart?
Activation of M2 muscarinic receptors by Ach release from parasympathetic nerves stimulates Gi which increases the open probability of K+. The increased K+ permeability causes hyperpolarisation slowing the intrinsic firing rate giving a negative chronotropic effect
How does Gs affect inotropy in the heart?
Activation of beta1 adrenoceptors increases the amount of VOCCs via Gs both directly and indirectly via cAMP and PKA which phosphorylates and activates VOCCs
How a g-proteins used in arteriolar vasoconstriction?
noradrenaline acts on alpha1 adrenoceptors to stimulate phospholipase C and IP3 production via Gq.
The IP3 releases ER Ca2+ and initiates a contractile response