Structure of BR (1BRX; ref. 10). (a) View from the cytoplasmic side along the membrane normal. (b) View from the C helix side along the membrane. The retinal chromophore is colored yellow in the space-filling model. Backbones of A–D helices (residues < 131) and E–G helices (residues ≥ 131) are colored blue and red, respectively. The side chains of the 11 threonines inside the membrane, at positions 17, 24, 46, 47, 55, 89, 90, 121, 142, 178, and 205, are represented by space-filling models. The other threonines are at positions 5, 67, 107, 128, 157, 170, and 247 in the loop and terminal regions.