which are true about g-protein coupled receptors and why?

Question

Which are true about g-protein coupled receptors?

unbound to ligand act as a GAP for the beta/gamma subunit

unbound to ligand act as a GAP for the alpha subunit

bound to ligand act as a GEF for the alpha subunit

bound to ligand act as a GEF for the beta/gamma subunit

interact with only stimulatory G-proteins

are an amplification step in a signaling pathway because they can interact with many G-proteins

can only interact with one G-protein per ligand that binds

Answer 1

Hi Brianna,

Here's a couple of tips that might help!

1. The alpha subunit of GPCRs has GTPase activity, while the beta and gamma do not.
2. This eliminates answers A and D
3. Ligand binding induces the exchange of GDP with GTP in the alpha subunit. Therefore, in the bound state, GPCRs are GEFs.
4. This makes answer C true.
5. GTP hydrolysis does not occur until after the G-protein has dissociated from the GPCR. Hydrolysis can be stimulated by "regulators of G protein signaling" (RGS). So RGS proteins act as GAPs.
6. This makes answer B false
7. There are many types of alpha subunits of G-proteins. These can be stimulatory or inhibitory.
8. This makes answer E false
9. It is generally thought that once ligand binds a GPCR and the G-protein dissociates, another G-protein can bind the same GPCR and become activated. This is one way signal amplification occurs.
10. This makes answer F true and G false.

Here's a summary!

a. unbound to ligand act as a GAP for the beta/gamma subunit - FALSE

b. unbound to ligand act as a GAP for the alpha subunit. - FALSE

c. bound to ligand act as a GEF for the alpha subunit. - TRUE

d. bound to ligand act as a GEF for the beta/gamma subunit. - FALSE

e. interact with only stimulatory G-proteins - FALSE

f. are an amplification step in a signaling pathway because they can interact with many G-proteins - TRUE

g. can only interact with one G-protein per ligand that binds - FALSE