Targeting RGS proteins to GIRK channels in a macromolecular complex may facilitate the RGS-mediated changes in the coupling efficiency between a GPCR and GIRK channel. Here, the coupling efficiency is determined by the EC50 concentration; that is, the agonist concentration required to activate 50% of the maximal GIRK current. Low coupling efficiency and therefore high EC50 reflect poor G-protein availability and low GIRK channel affinity for Gβγ dimers. The value of the EC50 depends on several additional parameters, including receptor number, receptor-ligand affinity, and agonist efficacy, which however remain constant under the conditions studied here.
