Receptor activation: what does the rhodopsin structure tell us?

G-protein-coupled receptors (GPCRs) are a large family of seven-transmembrane-helix proteins that mediate responses to hormones, neurotransmitters and, in the case of rhodopsin, photons. The recent determination of the structure of rhodopsin at atomic resolution opens avenues to a deeper understanding of GPCR activation and transmembrane signaling. Data from previous crosslinking, spin labeling and scanning accessibility experiments on rhodopsin have been mapped onto the high-resolution structure. These data correlate well and are consistent with the structure, and suggest that activation by light opens a cleft at the cytoplasmic end of the seven-helix bundle of rhodopsin. Furthermore, lessons learned from rhodopsin might also apply to other members of this essential family of receptors.