The arrangement ok alpha helices in rhodopsin

Purpose: Determine the arrangement of helices of the G protein-coupled receptor Rhodopsin. Methods: Rhodopsins are the only seven helix receptors available in milligram quantities for crystallization. Electron cryo-microscopy of two dimensional crystals was used to determine the three dimensional structure of bovine and frog rhodopsin. Results: In the three dimensional map seven regions of density are visible from which tilt angles of all seven helices can be deduced. Conclusions: We have determined a medium resolution three dimensional structure of rhodopsin. The helices are tightly packed on the intracellular side. Towards the extracellular (i.e. intradiscal) side a cavity formed mainly from heiices 3,4,5,6 and 7 was observed which is most likely occupied by the retinal. The observed transmembrane structural features of rhodopsin are likely to represent a model for other G protein-coupled receptors.