Photoreceptor membranes rhodopsin

In connection with the problem of oscillations discussed by previous speakers and other types of dynamical behavior of membranes, it would probably be timely to mention here in some more detail the experiments with vision rhodopsin that were performed in our institute by using the Mossbauer spectroscopy method [G. R. Kalamkarov et al., Doklady Biophys., 219, 126 (1974)]. These experiments manifested the existence of reversible photo-induced conformational changes in the photoreceptor membrane even at such low temperature as 77°K. We have labeled various samples of solubilized rhodopsin and of photoreceptor membranes by iron ascorbate enriched with Fe57 and looked for the change of Mossbauer spectra caused by the illumination of our samples. 

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. 

In contrast, proteins vary markedly in their lateral mobility. Some proteins are nearly as mobile as lipids, whereas others are virtually immobile. For example, the photoreceptor protein rhodopsin (Section 32.3.1). a very mobile protein, has a diffusion coefficient of 0.4 pm s f The rapid movement of rhodopsin is essential for fast signaling. At the other extreme is fibronectin, a peripheral glycoprotein that interacts with the extracellular matrix. For fibronectin, D is less than 10-4 pm2 s f Fibronectin has a very low mobility because it is anchored to actin filaments on the inside of the plasma membrane through integrin, a transmembrane protein that links the extracellular matrix to the cytoskeleton. 

Rodriguez de Turco EB, Deretic D, Bazan NG, et al. Post-golgi vesicles cotransport docosahexaenoyl-phospholipids and rhodopsin during frog photoreceptor membrane biogenesis. J Biol Chem 1997 272(161 10,491-10,497. 

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. 

Rhodopsin 348 38,892 Photoreceptor membrane protein in the retina of the eye. Contains 11-c/s retinal as chromophore... 

Each receptor cell consists of the outer segment, which contains the photoreceptor membranes and the inner segment, which contains the nucleus, mitochondria, and synaptic contacts. The receptor membranes consist of lamellar flattened membrane sacks (see Figure 46). There are slight differences in their morphologies in cones and rods. The photopigments (e.g., rhodopsin) are embedded in these sack membranes. 

INFLUENCE OF THE LIPID ENVIRONMENT ON THE PROPERTIES OF RHODOPSIN IN THE PHOTORECEPTOR MEMBRANE... 

In order to study the lipid-protein interaction in the photoreceptor membrane we have changed the micro-environment of the rhodopsin molecules by removal and replacement of lipids. Two different delipidation procedures have been applied 1. treatment of photoreceptor membranes with phospholipase-C (Borggreven et al., 1971), and 2. affinity chromatography over a concanavalin A-sepharose-4B column (van Breugel et al., 1976). 

Reconstitution of the delipidated photoreceptor membrane or rhodopsin preparations with selected phospholipids is achieved by shaking them together with additional DTAB for 1 h (Hong Hubbell, 1973). The detergent is then removed by the dialysis procedure described in the previous paragraph. Recovery of rhodopsin ranges from 80-100%. Freeze-fracture electronmicroscopy shows that rhodopsin is incorporated into lipid bilayer structures (Chen Hubbell, 1973). 

Fig. 7. Effect of treatment by pronase and phospholipase-C on the 500 nm absorption of rhodopsin in photoreceptor membranes ...

Phospholipase-C treatment of the photoreceptor membrane also has a proununced affect on the extent to which rhodopsin is attacked by proteolytic enzymes (van Breugel et al., 1975). In the case of untreated membranes rhodopsin is partially degraded by incubation with pronase without a marked loss of 500 nm absorbance (Fig. 7, bottom right). After prior treatment of the membranes with phospholipase-C there is a subtantial loss of 500 nm absorbance (Fig. 7, top left), and gel electrophoresis shows that this is accompanied by more extensive digestion of the rhodopsin molecule. Control experiments indicate that this is not due to the effect of phospholipase per se (Fig. 7, bottom left) and that the effect does not occur when phos-pholipase-C treatment follows that with pronase (Fig. 7, top right). These findings indicate that removal of phospholipids makes the rhodopsin molecule more vulnerable to attack by proteolytic enzymes. Thus rhodopsin must normally be deeply embedded in the hydrophobic core of the phospholipid bilayer with only a small part of the molecule exposed to the aqueous phase. 

The regeneration capacity of rhodopsin, defined as the amount of rhodopsin (in precent of the original amount) obtained after photolysis and reaction with excess 11-cis retinaldehyde, is 92% for native photoreceptor membranes (Table II, 3rd column). This parameter is very sensitive to detergent action. It is reduced to zero in preparations solubilized in DTAB (100 mM), but can recover nearly fully upon reconstitution with phospholipids and removal of detergent by dialysis. 

Fig. 8. Effect of delipidation on the photolytic sequence of rhodopsin. A native photoreceptor membrane suspension B suspension of delip-idated rhodopsin. Curves 1 show in both cases the rhodopsin spectrum before illumination. Curves 2 are taken 1 min after a 30-sec illumination. Curves 3 are taken after addition of 50 mM hydroxylamine to the illuminated preparations ( to convert all free retinaldehyde to retinylidene oxim).

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). 

When electromagnetic radiation with wavelengths between 800 and 400 nm hits the human retina, a chain of events is initiated which eventually results in the excitation of the visual nerve and the perception of light or colour in the brain. The molecular basis for this process is the photoreceptor protein rhodopsin, which forms part of the membrane layers inside the cone and rod cells of the retina. The... 

McDowell JH, Kiihn H (1997) Light-induced phosphorylation of rhodopsin in cattle photoreceptor membranes substrate activation and inactivation. Biochemistry 16 4054-4060... 

In summary, the body of evidence currently available for F. salina indicates that these cells possess two different types of putative photoreceptors, a rhodopsin-like and a hypericin-hke pigment. The hypothesis that a rhodopsin pigment is responsible for phototaxis is supported by several pieces of evidence based on the similarity of its action spectrum to that of P. bursaria, by the effect of metabohc inhibitors such as hydroxylamine and zaprinast, by gene analysis, and by the suggested effect of Hght on membrane conductance. 

Shevchenko, T.F., Kalamkarov, G.R., and Ostrovsky, M.A., The lack of H transfer across the photoreceptor membrane during rhodopsin photolysis. Sensory Systems (USSR Acad. Sci.), 1,117, 1987 (in Russian). 

Kiilm, H., HaU, S.W., and Wilden, U., Light-induced binding of 48-kDa protein to photoreceptor membranes is highly enhanced by phosphorylation of rhodopsin, FEBS Lett., 176, 473,1984. 

Moreover, efficient rhodopsin regeneration may precede enzymatic reduction of all-fran.v-retinal to all-trans-retinol in the aged retina (Figure 15.2c) (Schadel et al., 2003). Upon rhodopsin regeneration, all-trans-retinal is released from the exit site of the protein into the lipid membrane (Figure 15.2c) (Schadel et al., 2003). From here the removal of all-tnms-retinal to the outer leaflet of the disc membrane is dependent on activity of ATP-binding cassette trasporter A4 (ABCA4) present in the rim of photoreceptor disc, known also as ABCR protein. 

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

Multiple forms of heterotrimeric G proteins exist in the nervous system. Three types of heterotrimeric G protein were identified in early studies. G termed transducin, was identified as the G protein that couples rhodopsin to regulation of photoreceptor cell function (see Ch. 49), and Gs and G were identified as the G proteins that couple plasma membrane receptors to the stimulation and inhibition, respectively, of adenylyl cyclase, the enzyme that catalyzes the synthesis of cAMP (see Ch. 21). 

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