Membrane proteins, encoded by ~20% of genes in almost all organisms, including humans, are critical for cellular communication, electrical and ion balances, structural integrity of the cells and their adhesions, and other functions. Atomic-resolution structures of these proteins furnish important information for understanding their molecular organization and constitute major breakthroughs in our understanding of how they participate in physiological processes. However, obtaining structural information about these proteins has progressed slowly (1, 2), mostly because of technical difficulties in the purification and handling of integral membrane proteins. Instability of the proteins in environments lacking phospholipids, the tendency for them to aggregate and precipitate, and/or difficulties with highly heterogeneous preparations of these proteins isolated from heterologous expression systems have hindered application of standard structure determination techniques to these molecules.
Among membrane proteins, G-protein-coupled receptors (GPCRs)1 are of special importance because they form one of the largest and most diverse groups of receptor proteins. More than 400 nonsensory receptors identified in the human genome are involved in the regulation of virtually all physiological processes. Drug addiction, mood control, and memory (via 5-HT6 or neuropeptide receptors) are just a short list of processes in which GPCRs are critically implicated. Another even larger group of GPCRs consist of sensory receptors involved in the fundamental process of translation of light energy (rhodopsin and cone pigments), the detection of chemoattractant molecules, or the detection of compounds stimulating the taste buds (3, 4). The activity of GPCRs comes about when binding of diffusable extracellular ligands causes them to switch from quiescent forms to an active conformation capable of interaction with hundreds of G-proteins. Their roles as extracellular ligand-binding proteins make them attractive targets for drug design. GPCRs account for ~40% of all therapeutic intervention, and major GPCR research projects are found throughout the pharmaceutical industry (5, 6).
A paucity of structural data is available for GPCRs. The crystal structure of a member of the largest subgroup (I) of GPCRs, rhodopsin (7), and a ligand-binding domain of the metabotropic glutamate receptor with and without the ligand (8) have been determined recently. The data allow models, firmly based on the atomic-resolution structural information, to be further tested as to the conformational changes that these receptors undergo in going from the quiescent to the signaling state. In this article, we describe the further refinement of rhodopsin (7) and provide some clues about how the receptor could be activated by light.