Transmembrane signaling by GPCRs: Insight from rhodopsin and opsin structures

G-protein-coupled receptors (GPCRs), also known as seven-transmembrane (7TM) receptors, are the largest family of membrane proteins in the human genome. As versatile signaling molecules, they mediate cellular responses to extracellular signals. Diffusible ligands like hormones and neurotransmitters bind to GPCRs to modulate GPCR activity. An extraordinary and highly specialized GPCR is the photoreceptor rhodopsin which contains the chromophore retinal as its covalently bound ligand. For receptor activation the configuration of retinal is altered by photon absorption. To date, rhodopsin is the only GPCR for which crystal structures of inactive, active and ligand-free conformations are known. Although the photochemical activation is unique to rhodopsin, many mechanistic insights from this receptor can be generalized for GPCRs.