Three dimensional structure of rhodopsin obtained by electron cryo-microscopy

Purpose. To obtain 2-dimensional crystals of rhodopsin suitable for electron cryo-microscopy so that a 3-dimensional structure can be determined. Methods. Photoreceptor membranes containing frog rhodopsin were extracted with Tween detergents. Low dose electron cryo-microscopy was used to examine crystalline arrays of frog rhodopsin. Results. Membranes were produced that contained crystalline frog rhodopsin with p2 symmetry. Data were collected from 60 crystals with nominal tilt angles up to 40°. A map was calculated having a resolution limit of 7.5 Å horizontally and 16.5 Å normal to the plane. In the 3-dimensional map 7 regions of density are visible from which the approximate tilt angles of all 7 helices can be deduced. The overall tilt of the more tilted helices is unambiguously defined in the frog rhodopsin map. There is clearly additional density visible on one side of the membrane that cannot be explained by transmembrane helices; this additional density may represent a compactly folded extracellular (i.e. intradiscal) domain. On the intracellular side of the membrane, only little density is visible, mainly extending from one straight helix. Conclusions. We have determined the 3-dimensional structure of the photoreceptor protein rhodopsin, which is a member of the class of G-protein-coupled receptors (GPCR). For the first time, all 7 helices have been visualized directly. The protein forms a more compact helical bundle compared to bacteriorhodopsin (BR) and the tilts of the helices of frog rhodopsin are different from those of BR. The transmembrane structural features of rhodopsin are likely to represent a model for other GPCRs.