G Protein Coupled Receptors - G Protein Coupled Receptor Signalling

Front 1

What are secondary messengers?

Back 1

If the extracellular ligand e.g. Hormone is the primary messenger - the secondary messenger is a molecule produced by the activated receptor. They are therefore one of the initiating components of intracellular signal transduction cascades.

Front 2

What are two important secondary messengers?

Back 2

Cyclic AMP: 3'-5' cyclic adenosine monophosphate

IP3: Inositol-1,4,5-triophosphate

Front 3

What are GPCRs?

Back 3

G protein coupled receptors (GPCRs) are remarkably versatile signaling molecules. The members of this large family of membraneproteins are activated by a spectrum of structurally diverse ligands, and have been shown to modulate the activity of different signaling pathways in a ligand specific manner. They are coupled with trimeric GTPase complexes and have several transmembrane domains

Front 4

Why do GPCRs matter?

Back 4

They are receptors for many hormones- numerous drugs work by targeting them e.g. Salbutamol an asthma drug is an antagonist of beta adrenergic receptors

Front 5

What is the difference between small and large extracellular domains?

Back 5

GPCRs that bind to protein ligands have a large EC domain formed by part of a polypeptide chain. This domain and some transmembrane segments binds to the protein ligands. Receptors for small ligands e.g. Adrenaline have small EC domains and the ligand usually binds deep in the plane of the membrane to a site formed by amino acids from several transmebrane segments.

Front 6

What happens when extracellular signals bind to GPCR?

Back 6

Conformational change of the receptor in turn altering the conformation of the G protein

Front 7

What is the role of GS alpha?

Back 7

Activated enzymes adenyl cyclase which generates CAMP from ATP activating protein kinase A.

1) Ligand binds

2) G alpha S activated exchanging GDP - GTP

3) Adenyl cyclase is activated converting ATP to CAMP

4) PKA is activated: Has 4 subunits (2 regulatory which bind to AMP and 2 catalytic which phosphorylates the target)

Front 8

What are the levels of CAMP dependant on?

Back 8

The relative activity of adenylyl cyclase and CAMP phosphodiesterase - cell needs a way of switching off the CAMP

Front 9

How is GPCR signalling related to bacterial toxins?

Back 9

Cholera toxin catalyses transfer of ADP ribose from NAD+ to G alpha S.

- ADP ribolysation prevents the G alpha S from hydrolysing the GTP thus it is permanently on and stimulating adenylyl cyclase causing elevated levels of CAMP.

Front 10

Salbutamol is an asthma drug that binds to adrenaline receptors leading to CAMP generation - explain how it works

Back 10

Cyclic AMP generated downstream of the receptor. G alpha S subunit is activated and adenylyl cyclase converts ATP - CAMP. CAMP is a secondary messenger which activates PKA by binding to regulatory subunits.

Front 11

How do GPCRs increase cytosolic calcium to activate PKC?

Back 11

Activaed GPCR stimulates plasma membrane boundphospholipase via G protein. May be activated by the alpha subunit of Gq asshown by the BY subunits of another G protein or by both. 2 small intracellular messenger molecules areproduced when Pi (45) P2 is hydrolysed by active pLCB. IP3 diffuses through cytosol and releasescalcium from the ER by binding to and opening the IP3 gated calcium releasechannels. The large electrochemical gradient for calciumacross the membrane causes calcium to escape into the cytosol when the releasechannels are open. Diacylglycerol remains in the plasma membraneand together with phoaphatidylserine and calcium, helps to activte the proteinkinase C (PKC).

Front 12

What does IP3 do for us?

Back 12

Promotes the release of calcium from the ER to the cytosol. It binds to IP3R on the ER.

Front 13

How does calcium act as a signalling molecule?

Back 13

Ina resting cell, cytosdolic calcium levels are around 100nM, whereas in theintracellular calcium reservoirs – mainly the endoplasmic reticulum, calciumlevels are around 1mM. Stimulationof the cell with calcium mobilizing agonists leads to calcium release from theER . This activatescalcium binding proteins such as PKC; also calmodulin - a multifunctional calcium binding proteinwhich in turn activates numerous calcium sensitive processses · Thecalcium released from the endoplasmic reticulum drives gene expression; cellgrowth and differentiation; apoptosis.