Cell signalling 3: signalling through receptors I (Prof. Patel) Flashcards
What is the cascade involved in signalling through receptors ?
- An extracellular signalling molecule binds to a receptor protein
- The receptor activates intracellular signalling proteins
- These signalling proteins activate target proteins which can have different effects:
- metabolic enzyme –> altered metabolism
- gene regulatory protein –> altered gene expression
- cytoskeletal protein –> altered cell shape or mvnt
Do hydrophilic signalling molecules bind to cell-surface receptors or intracellular receptors?
Cell-surface receptors.
Only (small) hydrophobic molecules can bind intracelullar receptor, but they need carrier proteins to travel in the extracellular space and cytosol.
What are the 2 most common molecular switches ?
Phosphorylation (kinases/phosphatases) and GTP-binding proteins.
What are the 4 major second messengers ?
cAMP, cGMP, DAG and IP3.
How man GPCRs are there in the genome ?
What does this imply ?
About 800 –> common drug target
What kind of ligands may activate GPCRs ?
- hormones (e.g. AD)
- NTs (e.g. Glu)
- odorants
- photons
However, there are many “orphan” GPCRs.
Why were RJ Lefkowitz and BK Koblika awarded the Nobel Prize in Chemistry in 2012 ?
“[…] for studies of GPCRs.”
What do GPCRs couple to ?
Heterotrimeric G-proteins.
What downstream response can activated G-proteins elicit ?
- cAMP signalling
- IP3/DAG signalling
- MAPK signalling
- PtdIns 3-kinase signalling
- PLD signalling
- Redox signalling
- Cytoskeletal remodeling
- Ion channel modulation
What do GEFs do ?
They exchange GDP for GTP.
What do GAPs do ?
GAP = G-protein Activated Protein = GTPase-activating factor
GAPs hydrolyze GTP to GDP
What do GRKs (G-protein Receptor Kinases) do ?
GRKs phosphorylate the GPCR after the G-proteins have been activated and released –> prepares for Arrestin binding
What does RGS (Regalator of G-protein Signalling) do ?
It inactivates G-alpha (by promoting its hydrolysis) and G-beta/gamma.
What do Arrestins do ?
Arrestins bind to GPCRs to block further G protein-mediated signaling, target receptors for internalization, and redirect signaling to alternative G protein-independent pathways, such as β-arrestin signaling.
What was the first 2nd messenger discovered ?
Who discovered it ?
cAMP, discovered by EW Sutherland, who won the Nobel Prize in Medicine in 1971 “[…] for his discoveries concerning the mechanisms of the action of hormones.”
What does AC (adenylate cyclase) do ?
It converts ATP to cAMP.
How dos alpha-s act ?
Alpha-i ?
Alpha-s activates AC
Alpha-i inhibits AC
Which enzyme mediates the effects of cAMP ?
Most effects of cAMP –> mediated by PKA
How does cAMP act on PKA ?
What does this allow ?
It dissociates the C (catalytic) from the R (regulatory) subunits.
C subunits can now phosphorylate targets.
Which enzymes terminate cAMP signals ?
How do they do this ?
Phosphodiesterases (w/ H20) convert cAMP to 5’-AMP.
How does AD work on the B-adrenergic receptors in the heart ?
- AD binds to B-AR
- Galpha-s activates AC –> increase in cAMP
- cAMP release PKA C subunit
- PKA phosphorylates several proteins which increase Ca2+ levels
- increased Ca2+ levels increase force of contraction
How does alpha-q work ?
It activated PLC-beta, which produces DAG and IP3
DAG activates PKC and IP3 leads to Ca2+ release from ER
How does IP3 work ?
It activates intracelulallar l-gated ion channels (IP3R) on ER.
Many of the effects of Ca2+ are mediated by ______.
Calmodulin
What effect does Ca2+ binding have on calmodulin ?
It induces conformational changes and binding to target proteins.
What is CamKII ?
How does it function ?
CamKII = Ca2+-calmodulin dependant protein kinase II
- when calmodulin (bound to Ca2+) binds it, CamkII autophsphorylates (ATP–>ADP) and becomes fully active.
- when Ca2+ dissociates from calmodulin, the latter unbinds and CamkII becomes Ca2+ independant (50-80% active)
- when a phosphotase dephosphorylates CamkII, it becomes inactive
How are Ca2+ signals terminated ?
By Ca2+ pumps and exchangers (SERCA/PMCA).
What molecules mediates many of the effects of DAG ?
PKC
What are receptor tyrosine kinases (RTKs)?
Can you give any examples of such receptors ?
They are enzyme-linked receptors with intracellular tyrosine kinase domains which are activated by growth hormones:
- Insulin receptor, IGF-1 (insulin-like GF1) receptor
- EGF (Epidermal GF) receptor
- PDGF (Platelet-derived GF) receptor, CSF-1 receptor
- FGF (Fibroblast GF) receptor
How do receptor tyrosine kinases react upon activation ?
They auto-phosphorylate (form dimers) and recruit other proteins.
What are the different stages of the MAPK (Mitogen-Activated Protein Kinase Cascade) signalling cascade ?
Stimulus –> transducers –> MAPKKK –> MAPKK –> MAPK –> effectors –> cellular responses
What are the transducers of the MAPK pathway ?
- Grb-2 (adaptor) directly bound to the RTK
- Sos = a GEF bound to Ras
- Ras = monomeric G-protein
Give an example of a MAPK signalling cascade.
FGF –> Grb-2, Sos, Ras –> RAf –> MEK (or MAPKK) –> ERK (Extracellular-signal Regulated Kinase) –> Mesoderm inuduction
