Figure 1. Phototransduction. Rhodopsin (R) absorbs a photon (h) and becomes activated (R*). Photoactivated rhodopsin, R*, then activates transducin (T). Activated transducin (T*) causes an increase in the activity of phosphodiesterase (PDE to PDE*). The PDE* hydrolyses cyclic guanosine monophosphate (cGMP), causing closure of the cGMP-gated cation channel in the plasma membrane. From Hargrave and McDowell (1992). Copyright © 1992 Academic Press.

Figure 2. Diagram of the rod cell, the disc membrane containing rhodopsin, and the helix bundle model of rhodopsin. The rod cell of the retina is a highly specialized neuron whose outer segment consists primarily of a plasma membrane enveloping a stack of disc membranes. The disc membranes are 50% lipid and 50% protein. Most of the intrinsic membrane protein of the disc is rhodopsin, but a small amount of other proteins is found at the disc edges. An individual rhodopsin molecule is shown diagrammatically as an elongated transmembrane protein with seven transmembrane helical segments forming a bundle which provides a pocket for 11-cis retinal. From Hargrave et al. (1993). Copyright © 1993. Reproduced with permission of John Wiley & Sons, Inc.

Figure 3. Spectrum of rhodopsin and metarhodopsin II, and isomerization of retinal from 11-cis to all-trans by light. Rhodopsin (red line) has an absorbance due to retinal at 500 nm, and an absorbance at 280 nm due to opsin. Upon exposure to light, metarhodopsin II (with an absorbance due to retinal at 380 nm) is formed (green line). The spectrum was taken by Dr J. H. McDowell by dissolving bovine rod cell membranes in the detergent dodecylmaltoside. In the upper right-hand corner of the figure, protein-bound 11-cis retinal is isomerized by light to all-trans retinal.

Figure 4. The rhodopsin cycle. Rhodopsin (R) becomes photoactivated (R*) and activates transducin (T to T*). R* becomes phosphorylated by rhodopsin kinase (to R*P), and is inactivated by binding arrestin. Upon dissociation of all-trans retinal, R*P relaxes to phosphorylated opsin (OP) which is dephosphorylated by the phosphatase PrP2A. All-trans retinal is reduced, esterified and isomerized through a series of steps to form 11-cis retinal. Opsin rebinds 11-cis retinal, regenerating R. Adapted from Hargrave and McDowell (1992). Copyright © 1992 Academic Press.

Figure 5. Topographical model for bovine rhodopsin in the disc membrane. Rhodopsin's polypeptide chain consists of alternating predominantly hydrophilic and hydrophobic segments. It traverses the lipid bilayer seven times, exposing hydrophilic segments to the aqueous environment and burying hydrophobic helices (I–VII) in the lipid bilayer. The helix-connecting loops i1–i4 and the C-terminal sequence are located on the intracellular or cytoplasmic surface. The N-terminal sequence and loops e1–e3 are located on the extracellular surface (for rhodopsin molecules located in the outer segment plasma membrane) or on the intradiscal surface (for rhodopsin molecules in the disc membranes). Oligosaccharide chains are shown attached to asparagines 2 and 15. A disulfide bridge connects cysteines 110 and 187. Cysteines 322 and 323 are palmitoylated. 11-cis retinal is linked to lysine 296 in helix VII. From Hargrave and McDowell (1992). Copyright © 1992 Academic Press.

Figure 6. The seven helices of vertebrate rhodopsin. (a) Balsa-wood model representing the structure of frog rhodopsin obtained by electron cryomicroscopy (Unger et al., 1997). The upper view shows the model viewed from helix 2 towards helix 6, and the lower view shows the opposite surface, from helix 6 towards helix 3. The top of the model is the cytoplasmic surface and the bottom the intradiscal (or extracellular) surface. The model is constructed of 33 contour sections 0.2 nm apart. The central part of each helix is shown by a line extending from +1.2 to –0.8 nm, with 0 being the centre. Adapted from Unger et al. (1997). (b) Three slices through the electron density map of frog rhodopsin. Sections correspond to the model shown in Figure 5a. Sections are taken at +1.2 nm above the centre, at the centre and at –0.8 nm below the centre of the map. Helices 1–7 correspond to the rhodopsin sequence as shown in the topographic model in Figure 3. The least tilted helices (4, 6 and 7) are shown in grey, and each of the most tilted helices (1, 2, 3 and 5) has a different colour. The grid spacing is 1 nm. Adapted from Unger et al. (1997).