Rhodopsin disk membrane structure

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. 

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

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 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. 

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. 

Fig. 4.13 (A) Crystal structure of bovine rhodopsin viewed from a perspective approximately normal to the membrane. The polypeptide backbone is represented by a ribbon model (gray) and the retinylidine chromophore by a licorice model (black). The coordinates are Irran Protein Data Bank file IfSS.pdb [69]. Some parts of the protein that protrude from the phosopholipid bilayta- of the membrane are omitted fw clarity. (B) The 11-cfr-retinyUdine chromophore is attached to a lysine residue by a protonated Schiff base linkage. Excitation results in isomerization around the 11-12 bond to give an all-rrfl s structure. (C, D) Schematic depictions of a field of rhodopsin molecules in a rod cell disk membrane, viewed nmmal to the membrane. The short arrows in the shaded ovals represent the transition dipoles of individual rhodopsin molecules. (Each disk in a human retina contains approximately 1,000 rhodopsins.) The transition dipoles lie approximately in the plane of the membrane, but have no preferred orientation in this plane. A polarized excitation flash (horizontal double-headed arrow in C) selectively excites molecules that are oriented with their transition dipoles parallel to the polarization axis, causing some of them to isomerize and changing their absorption spectrum (empty ovals in D). (E) Smoothed records of the absorbance changes at 580 run as a function of time, measured with probe light polarized either parallel or perpendicular to the excitation [14]. The vertical arrow indicates the time of the flash. The absorbance change initially depends on the polarization, but this dependence disappears as rhodopsin molecules rotate in the membrane...

Photo-activated retina rhodopsin, a G-protein coupled receptor (GPCR), is a major component in purple membranes (PM). Due to the high structural content of helices in GPCR, GPCR rich PM disks align in the presence of a magnetic field... 

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