Rhodopsin composition

Berman, A.L., Svetashev, V.I., Rychkova, M.N. and Shnyrev, V.L. (1979). Lipid composition of the photoreceptor membranes of the retina and the temperature stability of rhodopsins in marine fish (In Russian). In Physiology and Biochemistry of Marine and Freshwater Animals (E.M. Kreps, ed.), pp.172-180. Nauka, Leningrad. 

TVurin, V.A., Berman, A.L., Ryzhova, M.P., Chelomin, V.P. and Korchagin, V.P. (1982). Fatty acid composition of phospholipids of the binary layer of photoreceptor membranes and aminophospholipids of rhodopsin microenvironment in cold and warm blooded vertebrates (In Russian). Zhumal Evolutsionnoy Biochimii i Physiologii 18,101-105. 

The opsin consists of protein (ca. 80-85% of which is rhodopsin), phospholipids and carbohydrates and contains very little cholesterol (1-3%) (for a review, see [6]). While the molecular weight (e.g., 40000 for bovine rhodopsin) [7], carbohydrate [8,9], lipid and amino acid [10-12] composition have been established for some rhodopsins, there is as yet no example of a visual pigment for which the full amino acid sequence is known. Only a quarter of about the 300 residues of rhodopsin have been sequenced [13,14], 39 residues at the N-terminus and 40 residues at the C-terminus. The structure of the moiety containing retinal, i.e., retinal-lysine-alanine, which is located in the carboxy-terminal region has, however, been elucidated ([15] see also [78] and references therein). The N-terminal residue was identified as acetylmethionine [16]. 

The lipid membrane provides shelter for membrane proteins to do their functions. However, instead of working alone, membrane proteins such as ion channels work together with the membrane, such that the lipid composition around the protein actually affects the activation and the functioning of the protein. This idea is largely the essence of the lipid raft model (8), which highlights the importance of lipids in a variety of cellular functions. It has been observed, for example, that rhodopsin, which is the light sensitive membrane protein, favors interactions with polyunsaturated lipids (9). 

A molecular model based on the structure of rhodopsin was developed concurrently to mutagenesis studies in order to visualize the environment of the ligand binding site. We have found that the low resolution structure of rhodopsin seiwes as a more versatile template for G protein-coupled receptors (GPCRs) than the coordinates of bacteriorhodopsin [7]. The A2A receptor model was composed in steps including construction and energy minimization of each hehx individually, composition of the helical bundle based on consideration of... 

The visual transduction pathway is the best characterized G-protein-coupled signal transduction system. Study of the visual receptor, rhodopsin, over the past several decades has made it the archetype of the growing superfamily of heptahelical G-protein-coupled receptors (reviewed in Litman Mitchell, 1996a). The preeminent position of rhodopsin in this important superfamily will likely increase with the recent publication of the three-dimensional structure of rhodopsin (Palczewski et al., 2000). Many neurotransmitter receptors, as well as the olfactory and taste receptors, are members of this superfamily. Therefore, the effect of lipid membrane composition on various steps in visual signaling will be reviewed in some detail in this chapter. Given the similarity in mode of signaling, the observations made for the vision system should be of general applicability to other members of this receptor superfamily. 

Fig. 4. Examples of the effects of acyl chain composition (white bars) and cholesterol (gray bars) on A eq for the MI-MII equilibrium of photolyzed rhodopsin at 37°C. Higher values of correspond to higher equilibrium concentrations of Mil, the state of photolyzed rhodopsin that participates in visual signal transduction by binding the visual G protein.

Gibson NJ, Brown MF. Lipid headgroup and acyl chain composition modulate the MI-MII equilibrium of rhodopsin in recombinant membranes. Biochemistry 1993 32 2438-2454. 

Fatty acid compositional changes in the retina of the rodent eye can also impact the development of vision, which, in turn, will affect behavior of the animal. Recent biochemical and biophysical studies have demonstrated how the response of rhodopsin to different membrane lipids may significantly affect vision. DHA is primarily associated with membrane proteins of the 7-TM motif (i.e., seven transmembrane spanning regions). Such 7- fM proteins are also associated with a G-protein-coupled event (activation of a phosphodiesterase which cleaves GTP to GDP). Rhodopsin is a typical 7-TM G-protein-coupled... 

Figures 4.1B and 4.2B show that the isomeric compositions between visual and archaeal rhodopsins differ not only for the 01=02 double bond, but also for the C6-C7 single bond. In visual rhodopsins, the C6-C7 bond is in a cis form, and the...

Other proteins that have activities that correlate with the mesomorphic tendencies of the lipid bilayer include the vertebrate photoreceptor protein rhodopsin (42) and a dolichylphosphomannose synthase (43). The paucity of other examples reflects the lack of systematic studies. Membrane protein reconstitutions are generally difficult to perform, especially if the lipid composition is to be varied, and, therefore, are unlikely to be undertaken without good reason. Studies of correlations with lipid mesomorphic tendencies, stimulated by research such as that reported here, are now under consideration by several biochemical groups. Certainly, much more work is needed in this area. 

The chemistry of ROS has been studied by several laboratories, and recently reviewed by Daemen (1973). Over 90% of the protein is rhodopsin, a photosensitive glycoprotein of molecular weight around 35,000, which is imbedded in a lipid bilayer. Phospholipids make up about 96% of the lipids of cattle ROS and cholesterol is the major component of the neutral lipid fraction. Phosphatidyl choline (PC) and phosphatidyl ethanolamine (PE) are the major phospholipids in all species examined, with phosphatidyl serine (PS), phosphatidyl inositol (PI), and sphingomyelin (SPh) present in lesser amounts (Anderson and Maude, 1972). Detailed analysis of the photoreceptor membranes of vertebrate species ranging from frogs to humans have revealed a fairly constant phospholipid class and protein composition (Basinger and Anderson, unpublished). 

---