Modulation of GPCR conformations by ligands, G-proteins, and arrestins.
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G-protein-coupled receptors (GPCRs) have traditionally been thought to adopt two conformations: the inactive unliganded conformation and the active ligand-bound conformation. Interactions with G-proteins in cells and membranes are known to modulate the affinity of the receptor for ligand and therefore the conformation of the receptor. Such observations led to the proposal of the ternary complex model. However, subsequent studies of constitutively active GPCRs led to the development of an extended version of this model to account for active conformations of the receptor in the absence of agonist. A significant difficulty with many of the studies, upon which this latter model was based, is the lack of knowledge of receptor and G-protein concentrations due to the two-dimensional nature of the membranes used to perform the measurements. Over the past decade, we have studied the interaction of GPCRs, G-proteins, arrestins, and ligands in solubilized systems, where the concentration of each component can be defined. Here we summarize results of these studies as they pertain to the regulation of GPCR conformations and affinities for interacting species.