Projection structure of rhodopsin.

Light absorption by the visual pigment rhodopsin triggers, through G-protein coupling, a cascade of events in the outer segment of the rod cell of the vertebrate retina that results in membrane hyperpolarization and nerve excitation. Rhodopsin, which contains 348 amino acids, has seven helices that cross the disk membrane and its amino terminus is extracellular. A wealth of biochemical data is available for rhodopsin: 11-cis retinal is bound to lysine 296 in helix VII; glutamic acid 113 on helix III is the counterion to the protonated Schiff's base; a disulphide bridge, cystine 110-187, connects helix III to the second extracellular loop e2 (refs 13, 14); the carboxy terminus has two palmitoylated cysteines forming a cytoplasmic loop i4 (ref. 15); three intracellular loops i2, i3 and i4 mediate activation of the heterotrimeric G protein transducin; glutamic acid 135 and arginine 136 at the cytoplasmic end of helix III affect binding of transducin. But to provide a framework to interpret these data, not only for rhodopsin but for other G-protein-coupled receptors, requires the structure to be determined. Here we present a projection map of rhodopsin showing the configuration of the helices.