Protein rhodopsin

FIGURE 18.36 The incorporation of retinal into the light-sensitive protein rhodopsin involves several steps. All- ram-retinol is oxidized by retinol dehydrogenase and then iso-merized to ll-cis-retinal, which forms a Schiff base linkage with opsin to form light-sensitive rhodopsin. 

Photosynthetic prokaryotes do not have chloroplasts. Their photosynthetic pigments are embedded in their cell walls. Some use bacteriochlorophyll for light harvesting. In the proteobacteria and archaea, light harvesting is accomplished by the protein rhodopsin, which acts as a photo-driven proton pump that fuels phosphorylation of ADP. 

S-prenyl Geranylgeranyl Famesyl heterotrimeric G-proteins (y-sub unit) Ras proteins rhodopsin kinase See chapter 5 C-terminus... 

In 1985 Tyminski etal. [55, 56] reported that two-component lipid vesicles of a neutral phospholipid, e.g. DOPC, and a neutral polymerizable PC, bis-DenPC (15), formed stable homogeneous bilayer vesicles prior to photopolymerization. After photopolymerization of a homogeneous 1 1 molar lipid mixture, the lipid vesicles were titrated with bovine rhodopsin-octyl glucoside micelles in a manner that maintained the octyl glucoside concentration below the surfactant critical micelle concentration. Consequently there was insufficient surfactant to keep the membrane protein, rhodopsin, soluble in the aqueous buffer. These conditions favor the insertion of transmembrane proteins into lipid bilayers. After addition and incubation, the bilayer vesicles were purified on a... 

Fig. 13. Freeze-fracture replicas of rhodopsin-polyl5/DOPC membranes in 30% glycerol/water frozen from room temperature. The particles are morphological manifestations of the protein rhodopsin. The nonrandom distribution of particles indicates the presence of enriched domains of lipid and of protein. The particle-free domains constitute about 30% of the surface area.

A stack of about 1 000 disks in each rod cell contains the lightsensing protein rhodopsin,10 in which the chromophore 11-o i-retinal (from vitamin A) is attached to the protein opsin. When light is absorbed by rhodopsin, a series of rapid transformations releases all-frans-retinal. At this stage, the pigment is bleached (loses all color) and cannot respond to more light until retinal isomerizes back to the 11 -cis form and recombines with the protein. 

THE SHAPE OF A SMALL ORGANIC MOLECULE, RETINAL. THIS FORCES A CHANGE IN THE SHAPE OF THE MUCH LARGER PROTEIN, RHODOPSIN, TO WHICH IT IS ATTACHED. THE CARTOON DRAWING OF THE PROTEIN IS NOT TO SCALE. 

Monomolecular films of the membrane protein rhodopsin have been investigated in situ at the air-water interface by PM-IRRAS and X-ray reflectivity in order to find conditions that retain the protein secondary structure [104]. The spreading of rhodopsin at 0 or 5 mN/m followed by a 30 min incubation time at 21 °C resulted in the unfolding of rhodopsin. In contrast, when spreading is performed at 5 or 10 mN/m followed by an immediate compression at, respectively, 4 or 21 °C, the secondary structure of the protein is retained. 

Information processing. Stimuli external to a cell, such as hormone signals or light intensity, are detected by specific proteins that transfer a signal to the interior of the cell. A well-characterized example is the visual protein rhodopsin, located in membranes of retinal cells. 

Great excitement has been generated in the signal transduction field by the first determination of the structure of a seven-transmembrane-helix receptor—the visual system protein rhodopsin—discussed in Chapters 15 and ... 

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. 

In the retina a protein called opsin combines with 11-cts-retinal to form a modified protein, rhodopsin. The ll-cis-retinal portion of rhodopsin is a prosthetic group (a nonprotein portion of a protein that is necessary for its action). 

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. 

Membrane structures that contain the visual receptor protein rhodopsin were formed by detergent dialysis on platinum, silicon oxide, titanium oxide, and indium—tin oxide electrodes. Electrochemical impedance spectroscopy was used to evaluate the biomembrane structures and their electrical properties. A model equivalent circuit is proposed to describe the membrane-electrode interface. The data suggest that the surface structure is a relatively complete single-membrane bilayer with a coverage of 0.97 and with long-term stability/... 

Q Wu and Stryer used FRET to estimate molecular dimensions of the visual pigment protein rhodopsin. They attached various fluorescent donor molecules at specific sites on the protein, and used the intrinsic retinal chromophore as the acceptor group. For one particular fluorescent donor, attached to site B on the protein, Rq was 5.2 nm and the observed fluorescence quenching was 36%. What is the distance between site B and the retinal chromophore ... 

The first step in the light-activated isomerization of retinal in the visual photoreceptor protein, rhodopsin, occurs in less than 6 ps. How far would a beam of light travel in this time ... 

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