Effect of GTP on ligand binding.. confused - (Nov/09/2011 )

Hi all,

Can you guys help me out with this?
Studies have shown that GTP analouges decrease the high affinity binding sites in binding assays. However, would'nt GTP coupling enhance high affinity state?

In my binding experiments I have a Kd of 14nM for my receptor "A", however,when I co-transfect a component "b" (which is a partner of receptor "A"), with my receptor " A", the Kd is 0.5nM and there is no change in potency (Ec50)!

Post doc in my lab says that my component B is possibly preventing GTP disassociation from the receptor "A"..

Is this a valid argument? does'nt the GTP binding itself dissociate the G-protein from the receptor?

Confused!! please help!!!!
Thanks

Just clarify a few things GTP does not bind to the receptor but to the G-protein.

In terms of GTP lower receptor affinity, this could be in the case in membrane binding preps. This is because receptors can exists in two states a low affinity sate (R) this is when the receptor is not coupled to the G-protein. When the receptor is coupled to its G-protein it is in a high affinity sate (R*), this is the inactive G-protein so to say when it is in association with GDP. So when you add GTP it causes the G-proteins to become activated so they are not associated with its receptor and so lowering affinity entering the R state.

There could be many explanations for what you are observing, do your see changes in the efficacy (Emax)?

I suggest reading this excellent paper on receptor theory

http://www.sciencedirect.com/science/article/pii/S0022519399909888#

Hope this helps