GTP binding is a method used to study distribution and functional activation of seven transmembrane, heterotrimeric G protein coupled family (GPCRs) receptors.
This assay works best with Gi-coupled receptors. Upon activation of the GPCR by an agonist, the receptor changes conformation exposing a binding site for a G-protein complex. Once this G-protein complex is bound, the Gα protein can release GDP and bind GTP. These types of assays are sometimes referred to as a GTP exchange assay. Because the GTP used in this assay is a radiolabeled non-hydrolyzable analog ([35S]-GTPγS, which is GTP labeled on the gamma phosphate with 35S), you will be able to measure the activation of the GPCR being studied by measuring the amount of [35S]-GTPγS bound to the receptor.
Functional Assays can be used to:
Determine the pharmacological values of potency (EC50) & efficacy (EMAX) of the ligand to activate the GPCR of interest.
Visualize regional differences of activated GPCRs by a ligand using Functional Autoradiography.
Determine if the ligand is a full agonist, partial agonist, inverse agonist or antagonist.