Disk membrane, rhodopsin

Rod outer segment membranes are arranged in stacks of disks containing rhodopsin 809... 

Asymmetric distribution of phospholipids across the retinal rod outer segment disk membrane has been shown to be associated with light reception by rhodopsin. It is known that the major phospholipids of this membrane, phosphatidylcholine and phosphatidylethanolamine, are symmetrically distributed across the membrane in the dark but not... 

Rhodopsin and transducin are embedded in the disk membranes in the outer rod segment. 

Sitaramayya, A. Rhodopsin kinase prepared from bovine rod disk membranes quenches hght activation of cGMP phosphodiesterase in a reconstituted system. Biochemistry, 25, 5460-5468 (1986)... 

In its excited conformation, rhodopsin interacts with a second protein, transducin, which hovers nearby on the cytoplasmic face of the disk membrane (Fig. 12-33). Transducin (T) belongs to the same family of hetero-trimeric GTP-binding proteins as Gs and G,. Although... 

The formation of metarhodopsin II is fast enough to be an obligatory step in visual transduction. It clearly is associated with changes in the interactions between rhodopsin and its surroundings. A reasonable hypothesis, therefore, is that the changes in protein structure allow metarhodopsin II to initiate an interaction with some other component of the disk membrane. We explore the nature of this component in the following sections. 

The disk membranes of the rod outer segment contain a phosphodiesterase that hydrolyzes cGMP to 5 -GMP. If the membranes are kept in the dark, the phosphodiesterase remains in a relatively inactive state and the cGMP content of the cell is high. When the cell is illuminated and rhodopsin is converted to metarhodopsin II, the activity of the phosphodiesterase increases, initiating a drop in the cGMP content. The reduced cGMP concentration decreases the Na+ permeability of the plasma membrane (see fig. S2.ll)... 

Dratz, E. Hargrave, P. (1983) The structure of rhodopsin and the rod outer segment disk membrane Trends Biochem Sci vol. 8, pp 128-131... 

The biochemistry that mediates photon absorption in the disk membranes and closing of the ionic channels in the plasma membrane is summarized in Fig. 2 and discussed in detail in reviews that provide access to the original literature (Helmreich and Hofmann, 1996 Menon et al, 2001 Hamm 1998, 2001 Bunemann and Hosey, 1999 Krupnick and Benovic, 1998). In the dark (inactive) state, rhodopsin (R) contains a covalently bound 11 -cis retinal chromophore buried within the protein core. In rhodopsin, the chromophore absorbs maximally around 500 nm, and absorption of a photon isomerizes the retinal to the all-trans form within about 200 fs (Peteanu et al, 1993) with a quantum efficiency of 0.67 (Dartnall, 1972). The change in chromophore shape creates strain, and subsequent thermal relaxation of both chromophore... 

Fig. 17A (see color insert) shows a ribbon model of the rhodopsin structure indicating the residues assigned to the interface in each helix by a sphere centered on the corresponding of-carbon. Also shown is a sphere on the a-carbon of residue 314, which is located in the interface (see Section III,F). Clearly, these residues define a unique plane of intersection of the molecule with the membrane-aqueous interface. The shaded band in Fig. 17 represents a phospholipid bilayer with a phosphate-phosphate distance of 40 A, the expected thickness of the bilayer in the disk membrane (Saiz and Klein, 2001). The outer interface of the bilayer is positioned so that the polar head groups coincide with the intersection plane defined by the data in Fig. 16. This procedure then fixes the intersection plane of the molecule on the extracellular surface as well.

An alternative and more interesting model is the formation of rhodopsin dimers across the disk membranes, as shown in Fig. 18B. The putative tail-to-tail dimer interface is formed by side-by-side stacking of the 1 and f) 2 strands from each molecule to make a continuous antiparallel /J-sheet. The Pro-16-Pro-23 loops from each monomer interdigitate and lay over one surface of the sheet. Intermolecular salt bridges and Ca2+ binding sites formed from pairs of carboxylate residues, one from... 

An additional basic difference between rhodopsin and bacter-iorhodopsin is associated with the structure of the pigment in the membrane. In contrast to the well-established rotational and translational mobility of the rhodopsin molecule in the viscous disk membrane (31-35), X-ray diffraction methods have shown that bacteriorhodopsin in the purple membrane is organized as a rigid two-dimensional hexagonal lattice with a 63 A unit cell (36-38). ... 

By comparing the BAT absorbance in a frog retina with that in a suspension of rod outer segments, Yoshizawa and co-workers concluded that the angle between the retinal transition moment of bathorhodopsin and the disk membrane plane is considerably smaller than that of rhodopsin (0° vs 18° respectively) (327,328). Such a change in geometry is consistent with an all-trans structure in BAT, but not with a simple proton translocation. 

Opsin can be considered to be a retinaldehyde receptor protein, functioning in the same way as cell surface receptor G-proteins (Sakmar, 1998). Like receptor proteins, opsin is a transmembrane protein with seven a-helical regions in the transmembrane domain the difference is that opsin spans the intracellular disk membrane of the rod or cone cell, whereas hormone and neurotransmitter receptors span the plasma membrane of the cell. The response time of rhodopsin is considerably faster than that of ceU surface receptor proteins. 

Diagrammatic representation of bovine rhodopsin embedded in the disk membrane. Only three of the seven a-helical segments are shown. The only point of attachment between retinaldehyde and opsin is the indicated aldimine bond. [Modified and reproduced, with permission, from D, F. O Brien, The chemistry of vision. Science 218,961 (1982), 1982 by the American Association for the Advancement of Science.]... 

The regeneration of rhodopsin following exposure to bright light requires the following steps. The a -trans retinaldehyde combines with phosphotidylethanolamine (PE) to form a protonated Schiff base N-retinylidene-PE. This complex is then exported out of the disk by an ATP-binding cassette (ABC) transporter exclusively located in the retinol disk membrane (ABCR). Outside the disk the a -trans retinaldehyde is reduced to aW-trans retinol hy a dehydrogenase and transported to retinal... 

Straume M, Litman BJ. Equilibrium and dynamic bilayer structural properties of unsaturated acyl chain phosphatidylcholine-cholesterol-rhodopsin recombinant vesicles and rod outer segment disk membranes as determined from higher order analysis of fluorescence anisotropy decay. Biochemistry 1988 27 7723-7733. 

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