cell-surface receptors

Question 1

types

Answer 1

• GPCR
• ion channel-linked receptor
• enzyme linked receptor (TKR)

Question 2

g-protein coupled receptor

Answer 2

• Participate in processes responsible for vision, smell, taste and neuronal transmission in response to photons or binding of ions, hormones, peptides, chemokines and other factors.
• GPRs are molecular targets for many of currently used drugs  30% of commercially available drugs have GPRs as target.
• GPRs share a common seven-transmembrane α-helical bundle structure but can accommodate thousands of different ligands.
• Large amount of receptor diversity, but common mechanism of action
• Transmit signals to intracellular targets via G proteins
• Targets are plasma membrane (PM) bound enzymes or ion channels

Question 3

GPCR - general mechanism of action

Answer 3

1. Binding of ligand to extracellular domain of GPRs induces conformational change that allows cytosolic domain of the receptor to bind to inactive G protein at inner face of PM.
2. This interaction activates the G protein, which dissociates from the receptor
3. Activated G protein alfa subunit can now bind GTP instead of GDP, causing dissociation into activated alfa vs. beta/gamma subunits. Each of these can go on to activate target proteins.

Question 4

alfa-subunit

Answer 4

• a monomeric GTPase
• is active when bound to GTP which happens after a ligand binds to teh GPCR and a GEF (guanine nucleotide exchange factor) changes the GDP for GTP
• is inactive when bound to GDP which happens after GTP is hydrolysed by a GAP (GTPase activating protein)

Question 5

Gs - mechanism

Answer 5

1. binding of ligand to GPCR
2. activation of Gs protein subunits
3. alfa-unit activates adenylyl cyclase
4. ATP –> cAMP
5. cAMP activates PKA
6. PKA activates CREB
7. CREB binds CBP and causes transcription of CREB-binding element containing genes

Question 6

Gi - mechanism

Answer 6

• opposite of Gs –> deactivates adenylyl cyclase

- so: no cAMP, no active PKA, no active CREB, no transcription of target genes

Question 7

Gq - mechansim

Answer 7

1. binding of ligand to GPCR
2. activation of Gq protein subunits
3. alfa-unit activates phospholipase C-beta
4. PIP2 –> IP3 and diacylglycerol
5. IP3 opens IP3-gated calcium channels on ER
6. calcium together with diacylglycerol activate PKC
7. PKC has many effects
   - the calcium also activates calmodulin which activates CaM kinase which has many effects too

Question 8

enzyme-linked receptors

Answer 8

• Tyrosine kinase-linked receptors (TKRs):
  • Cell surface receptors that are directly linked to intracellular enzymes (kinases).
  • Includes receptors for most growth factors (NGF, EGF, PDGF), insulin, and Src.
  • Common structure: N terminal extracellular ligand-binding domain, single TM domain, cytosolic C-terminal domain with tyrosine kinase activity.
• Proteins that interact with the TKRs need to have SH2 or SH3 domains
• Src homology 2 (SH2) domains on other proteins bind to phosphotyrosine containing regions of TKRs resulting in:
  • localization of SH2-containing proteins at plasma membrane.
  • association of SH2-containing proteins with other proteins.
  • phosphorylation of SH2-containing proteins.
  • activation of enzymatic activity of SH2-containing proteins.

Question 9

TKR activation

Answer 9

1. ligand binding induces receptor dimerization (receptor crosslinking).
2. dimerization leads to autophosphorylation of the receptor (cross phosphorylation).
3. phosphorylation increases kinase activity & also creates specific new binding sites.
4. proteins that bind to these new binding sites transmit intracellular signals.

Question 10

RAS signalling

Answer 10

1. Activated TRK activates RAS GEF(GTPase)
2. This GEF activates Ras by exchanging its GDP with GTP
3. Ras –> MAP kinase kinase kinase (Raf)
4. Raf –> MAP kinase kinase (Mek) via phosphorylation
5. Mek –> MAP kinase (Erk)
6. Erk has many downstream targets

Question 11

PI3K signalling

Answer 11

1. survival signal binds to TKR
2. activated TKR activates PI3K
3. PIP2 –> PIP3 (phosphorylation)
4. PIP3 activates Akt and PDK1 (PDK1 also activates Akt)
5. Akt causes increased growth and apoptosis/autophagy inhibition