PROJECTION STRUCTURE OF RHODOPSIN

LIGHT absorption by the visual pigment rhodopsin1,2 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 excitation3-5. Rhodopsin, which contains 348 amino acids6-8, has seven helices that cross the disk membrane 69 and its amino terminus is extracellular. A wealth of biochemical data is available for rhodopsin: 11-cis retinal is bound10 to lysine 296 in helix VII; glutamic acid 113 on helix III is the counterion to the protonated Schiffs base11,12; 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 transducin16,17; glutamic acid 135 and arginine 136 at the cytoplasmic end of helix III affect binding of transducin18. 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.