The visual pigment rhodopsin is a member of the G protein-coupled receptor family. Electron cryo-microscopy was used to determine the three-dimensional structure of bovine rhodopsin from tilted two-dimensional crystals embedded in vitrified water. The effective resolution in a map obtained from the 23 best crystals was about 9.5 A horizontally and approximately 47 A normal to the plane of the membrane. Four clearly resolved tracks of density in the map correspond to four alpha-helices oriented nearly perpendicular to the plane of the membrane. One of these helices appears to be more tilted than anticipated from the projection structure published previously. The remaining three helices are presumably more highly tilted, given that they form a continuous "arc-shaped" feature and could not be resolved to the same extent. The overall density distribution in the low resolution map shows an arrangement of the helices in which the "arc-shaped" feature is extended by a fourth, less tilted helix. The band of these four tilted helices is flanked by a straight helix on the outer side and a pair of straight helices on its inner side.
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