Rhodopsin is the dim-light photoreceptor, and is a prototypi-cal member of the G protein-coupled receptor (GPCR) family. It consists of a 348-amino-acidprotein, opsin, which binds the chromophore 11-cis-retinalvia a protonated Schiff base linkage to Lys296, giving theground state of the protein an absorption maximum at498 nm. An equilibrium is formed betweenthe later photointermediates, metarhodopsin I (meta I, 480 nm) and metarhodopsin II (meta II, 380 nm). MetaII corresponds to the fully activated receptor, which binds toand activates the heterotrimeric G protein transducin.