Structure of the rhodopsin dimer: a working model for G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) participate in virtually all physiological processes. They constitute the largest and most structurally conserved family of signaling molecules. Several class C GPCRs have been shown to exist as dimers in their active form and growing evidence indicates that many, if not all, class A receptors also form dimers and/or higher-order oligomers. High-resolution crystal structures are available only for the detergent-solubilized light receptor rhodopsin (Rho), the archetypal class A GPCR. In addition, Rho is the only GPCR for which the presumed higher-order oligomeric state has been demonstrated, by imaging native disk membranes using atomic force microscopy (AFM). Based on these data and the X-ray structure, an atomic model of Rho dimers has been proposed, a model that is currently scrutinized in various ways. AFM has also been used to measure the forces required to unfold single Rho molecules, thereby revealing which residues are responsible for Rho's stability. Recent functional analyses of fractions from solubilized disk membranes revealed that higher-order Rho oligomers are the most active species. These and other results have enhanced our understanding of GPCR structure and function.