Unger V M, Hargrave P A, Baldwin J M, Schertler G F
MRC Laboratory of Molecular Biology, Cambridge, UK.
Nature. 1997 Sep 11;389(6647):203-6. doi: 10.1038/38316.
Rhodopsins, the photoreceptors in rod cells, are G-protein-coupled receptors with seven hydrophobic segments containing characteristic conserved sequence patterns that define a large family. Members of the family are expected to share a conserved transmembrane structure. Direct evidence for the arrangement of seven alpha-helices was obtained from a 9A projection map of bovine rhodopsin. Structural constraints inferred from a comparison of G-protein-coupled receptor sequences were used to assign the seven hydrophobic stretches in the sequence to features in the projection map. A low-resolution three-dimensional structure of bovine rhodopsin and two projection structures of frog rhodopsin confirmed the position of the three least tilted helices, 4, 6 and 7. A more elongated peak of density for helix 5 indicated that it is tilted or bent, but helices 1, 2 and 3 were not resolved. Here we have used electron micrographs of frozen-hydrated two-dimensional frog rhodopsin crystals to determine the structure of frog rhodopsin. Seven rods of density in the map are used to estimate tilt angles for the seven helices. Density visible on the extracellular side of the membrane suggests a folded domain. Density extends from helix 6 on the intracellular side, and a short connection between helices 1 and 2, and possibly a part of the carboxy terminus, are visible.
视紫红质是视杆细胞中的光感受器，属于G蛋白偶联受体，具有七个疏水片段，包含定义一个大家族的特征性保守序列模式。该家族成员预计共享保守的跨膜结构。从牛视紫红质的9埃投影图中获得了七个α螺旋排列的直接证据。通过比较G蛋白偶联受体序列推断出的结构限制，用于将序列中的七个疏水片段与投影图中的特征进行匹配。牛视紫红质的低分辨率三维结构和蛙视紫红质的两个投影结构证实了三个倾斜度最小的螺旋（4、6和7）的位置。螺旋5的密度峰更细长，表明它是倾斜或弯曲的，但螺旋1、2和3未解析出来。在这里，我们使用冷冻水合二维蛙视紫红质晶体的电子显微镜照片来确定蛙视紫红质的结构。图中的七个密度棒用于估计七个螺旋的倾斜角度。膜外侧可见的密度表明存在一个折叠结构域。密度从细胞内侧的螺旋6延伸，螺旋1和2之间的短连接以及可能的羧基末端的一部分可见。
