GPCR- signaling Flashcards
Does a GPCR function as a GEF or GAP? Explain why.
GPCRs function as GEFs because the GPCR induces alpha-subunit of the G-protein to release GDP, allowing GTP to bind. (GEF= Guanine nucleotide exchange factor, activate by promoting release of bound GDP)
An inactive GPCR is activated by binding to a ligand. The result is a G-protein with two activated components. Explain how this is realized.
- Ligand binds GPCR
- GPCR undergoes conformational change, acts as GEF, activating alpha subunit of G-protein, causing conformational change, release of alpha subunit from beta and gamma subunits.
- downstream relay of pathway by alpha subunit and beta-gamma subunit.
Explain how GPCR signaling activates protein kinase C and mobilizes calcium.
GPCRs activate inositol phospholipid signaling pathway through G protein q.
Gq activates phospholipase C-b (PLCbeta) which cleaves PI(4,5)P2 into IP3 and diacylglycerol.
IP3 diffuses to ER ad opens IP3-gated Ca2+ release channels, release of Ca2+ from ER.
Diacylglycerol activates protein kinase C.
Explain how GPCR signaling is involved in the production of cAMP and activation of transcription factor CREB.
Ligands binding GPCRs coupled to a stimulatory G-protein (Gs), activated alpha-subunit of Gs binds and activates adenylyl cyclase which causes an increase in cAMP concentration.
cAMP activates PKA, released activated catalytic subunits of PKA phosphorylate CREB, which is now activated and stimulates transcription of target genes.
How is a G-protein activated?
Ligand binding to GPCR causes conformational change, the activated GPCR acts as GEF and thereby induces the release of GDP from the alpha subunit of the G-protein and binding of GTP. Binding of GTP in the alpha subunit of the G-protein causes this subunit to be activated and released from the beta and gamma subunits.
Why is calcium release always maximal in GPCR signaling?
Low concentration of Ca2+ in cytosol, high concentration in ER, causing large gradient driving Ca2+ in cytosol when channels are opened.
IP3 opens channels, causing release. Ca2+-induced calcium release (CICR) causes even more release of calcium when channels are opened, causes maximal release.
Describe the three models of adaptation in the desensitization of GPCR.
- Receptor sequestration: GPCRs are temporarily moved to the interior of the cell (internalized) so they have no longer access to ligand.
- Receptor down-regulation: GPCRs are destroyed in lysosomes after internalization.
- Receptor inactivation: GPCRs become altered so they cannot interact with G proteins.
How does the arrestin protein contribute to the desensitization process of GPCR?
Activated GPCR stimulates GPCR kinase (GRK) to phosphorylate the GPCR on multiple sites.
Arrestin protein binds phosphorylated GPCR.
This prevents GPCR from interacting with G-protein. And serves as adaptor to help couple GPCR to clathrin-dependent endocytosis machinery -> inducing receptor mediated endocytosis.
What is the difference between the homologous and heterologous pathway in relation to the desensitization of GPCR?
In homologous desensitization only the activated GPCRs are d4esensitized.
In heterologous desensitization the activation of one GPCR can result in the desensitization of another GPCR.
What is the outcome of the desensitization of GPCR?
Desensitization of GPCRs causes the GPCRs to not be able to interact with ligands anymore, so the signal will no longer be relayed forward.
What is the difference between the action of second-messenger-dependent protein kinases and G-protein-coupled receptor kinases?
G-protein-coupled receptor kinases can only induce one cellular response after binding of a ligand. While second-messenger-dependent protein kinases can trigger many cellular responses after binding of a single ligand.
