Rhodopsin: A Prototypical G Protein-Coupled Receptor

Thomas P. Sakmar    The Howard Hughes Medical Institute, Laboratory of Molecular Biology and Biochemistry, Rockefeller University,New York, New York 10021

I Structure and Function of Rhodopsin: A Prototypical G Protein-Coupled Receptor

Visual pigments comprise a branch of a large family of G protein1-coupled receptors (1). Although they share many similarities with other G protein-coupled receptor types, there is significant specialization in visual pigments not found in other receptor families. In particular, pigments are made up of opsin apoprotein plus chromophore. The chromophore is a cofactor and not a ligand in the classical sense because it is linked covalently via a protonated Schiff base bond to a specific lysine residue in the membrane-embedded domain of the protein (Fig. 1). One of two retinoids serve as the chromophore for nearly all visual pigments. The chromophore in most vertebrate pigments is the aldehyde of vitamin A, 11-cis-retinal (Fig. 2). The chromophore in many invertebrate, fish, and amphibian pigments is the aldehyde of vitamin A2, 11-cis-3-dehydroretinal, which contains an additional carbon-carbon double bond in the β-ionone ring. An important structural feature of the retinal chromophore in rhodopsin, in addition to its Schiff base linkage, is its extended polyene structure, which accounts for its visible absorption properties and allows for resonance structures (2).

Rhodopsin has a broad visible absorption maximum (λmax) at about 500 nm. On photoisomerization of the chromophore, the pigment is converted to metarhodopsin II (MII) with a λmax value of 380 nm (Fig. 2). The MII intermediate is characterized by a deprotonated Schiff base chromophore linkage. MU is the active form of the receptor (R*, light-activated rhodopsin), which catalyzes guanine nucleotide exchange by transducin.

Photoisomerization of the 11-cis to all-trans form of the retinylidene chromophore is the primary event in visual signal transduction, and it is the only light-dependent step (Fig. 2). Chromophore isomerization activates the pigment, allowing it to interact with a specific heterotrimeric G protein, trans-ducin. In the case of the vertebrate visual system, transducin activation leads to the activation of a cyclic-GMP phosphodiesterase, and the closing of cyclic-GMP-gated cation channels in the plasma membrane of the rod cell. Light causes a graded hypeipolarization of the photoreceptor cell. The amplification, modulation, and regulation of the light response is of great physiological importance and has been discussed in detail elsewhere (3–5). However, it should be pointed out that despite the fact that the visual system functions over about a 106-fold range of light intensity, the retinal rod cell has single-photon detection capability due to extremely low levels of dark noise in rhodopsin and a significant degree of biochemical amplification. Thermal isomerization in a single rhodopsin molecule at physiologic temperature has been estimated to occur about once in 470 years (6). Activation of a single rhodopsin molecule by a single photon can prevent the entry of as many as 108 cations into the rod cell. The possibility of single-pheromone molecule detection by insect olfactory systems notwithstanding, the visual system is unique among sensory signal transduction systems in that it can detect single events.

The visual pigments of many species of vertebrates and invertebrates have been studied by absorption spectroscopy or microspectrophotometry of visual organs. Therefore, historically vertebrate visual pigments have been generally classified based on the photoreceptor cell type of the retina in which they are found (7). Rod cells, responsible for dim-light vision, contain rhodopsin (“red” opsin). Cone cells, responsible for bright-light and color vision, contain iodopsins (“violet” opsins), also known as cone pigments or color vision pigments. The cloning of opsins from a variety of species has allowed more detailed comparisons and phylogenetic classifications based on structural, spectral, and biochemical properties of visual pigments (8). The homology in the opsin family of genes indicates that divergent evolution occurred from a single precursor retinal-binding protein to form long- and short-wavelength-absorbing prototypes. The long-wavelength prototype diverged to form red and green pigments. The short-wavelength prototype then diverged to form a blue pigment and the family of rhodopsins and rhodopsin-like green pigments.

Each visual pigment is tuned to a particular wavelength of maximal absorption, λmax, although the visible absorbance peak tends to be quite broad. Even though a protonated retinylidene Schiff base chromophore is likely to be universal in visual pigments, the λmax values of visual pigments span the visible spectrum (i.e., from near-ultraviolet at about 400 nm to far visible red at about 600 nm). Distinct chromophore–protein interactions are responsible, directly or indirectly, for spectral tuning in visual pigments. Thus, differences in primary structure result in differences in spectral properties.

Bovine rhodopsin is the most extensively studied G protein-coupled receptor. A large amount of pigment (0.5–1.0 mg) can be obtained from a single bovine retina by sucrose density gradient...