Cell membrane Rhodopsin

Figure 3.3 Molecular structure of G-protein-coupled receptors. In (a) the electron density map of bovine rhodopsin is shown as obtained by cryoelectron microscopy of two-dimensional arrays of receptors embedded in lipid membrane. The electron densities show seven peaks reflecting the seven a-helices which are predicted to cross the cell membrane. In (b) is shown a helical-wheel diagram of the receptor orientated according to the electron density map shown in (a). The diagram is seen as the receptor would be viewed from outside the cell membrane. The agonist binding pocket is illustrated by the hatched region between TM3, TM5 and TM6. (From Schertler et al. 1993 and Baldwin 1993, reproduced from Schwartz 1996). Reprinted with permission from Textbook of Receptor Pharmacology. Eds Foreman, JC and Johansen, T. Copyright CRC Press, Boca Raton, Florida...

By far the most studied family of the G-protein-coupled receptors are the rhodopsin-like receptors. These are also the largest group of receptors in number as they include receptors not only for the monoamines, nucleotides, neuropeptides and peptide hormones, but they also include the odorant receptors which number several hundreds of related receptors. These receptors have short N-termini, a conserved disulphide bridge between the TM2-TM3 and TM4—TM5 extracellular domains, and variable-length C-termini. In some cases the C-terminus is myristolyated which by tying the C-terminus to the cell membrane generates a fourth intracellular loop. 

Alkyl chain heterogeneities cause cell membrane bilayers to remain in the fluid state over a broad temperature range. This permits rapid lateral diffusion of membrane lipids and proteins within the plane of the bilayer. The lateral diffusion rate for an unconstrained phospholipid in a bilayer is of the order of 1 mm2 s 1 an integral membrane protein such as rhodopsin would diffuse 40nm2 s 1. 

The effect of absorption of a quantum of light by rhodopsin is to cause an influx of some 105 Na+ ions. It can be surmised that the rhodopsin molecule normally blocks ion transport across the cell membrane, but its change of shape as free opsin leaves a pore which allows ion conduction. 

The PI anchor maintains adhesion of acetylcholinesterase (of the red blood cell) and of some proteoglycans (su I fated proteinsof the extracellular matrix) to the cell membrane Palmitic acid is bound via thiol-ester bonds toCys 322 and Cys 323 of rhodopsln (see the section on vitamin A), a 327-amino-add protein. The polypeptide chain of rhodopsin loops in and out of the membrane several times, leaving the possible function of the lipid as an anchor in question. Myristic add is bound to the catalytic subunit of the cAMP-dependent protein kinase, though this protein is cytosolic and soluble. 

The visual transduction system of retinal membranes contains a plethora of prenylated proteins. One of the prenylated components is a protein called transducin. This protein is a member of a family of G proteins that contain three distinct subunits (a, P, and y) and function in mediating signal transduction from cell membrane receptors to downstream effector proteins. In the case of visual transduction, photoactivation of rhodopsin leads to a conformational change that is sensed by transducin. In the absence of photoactivation of rhodopsin, transducin exists in a resting state in which Gt)P is bound to its a-subunit. Photoactivated rhodopsin catalyzes the exchange of GDP bound to transducin with GTP. This GTP-bound activated transducin then stimulates the enzymatic activity of another membrane-bound protein termed cyclic GMP phosphodiesterase. The latter... 

Structural changes in metarhodopsin It compared to rhodopsin are sensed by a heterotrimeric G protein called trans-ducin. The Ga subunit of transducin activates a cGMP phosphodiesterase that hydrolyzes cGMP to GMP The resulting decrease in cGMP concentrations in the rod cell leads to closure of a cGMP-gated ion channel. This leads to hyperpolarization of the rod cell membrane and initiation of a nerve impulse. 

Special considerations for membrane protein expression in E. coli include issues of targeting ideally the expressed protein can be incorporated into the bacterial cell membrane, allowing extraction of folded samples from detergent-solubilized cell membranes [34—37]. This has been the case for the small number of polytopic helical membrane protein structures that were successfully determined by solution NMR, namely diacylglycerol kinase (DAGK) [38], the disulfide bond isomerase DsbB [39], sensory rhodopsin II (pSRII) [40], and the mitochondrial uncoupling... 

About 57 per cent of the photons that enter the eye reach the retina the rest are scattered or absorbed by the ocular fluid. Here the primary act of vision takes place, in which the chromophore of a rhodopsin molecule absorbs a photon in another n-to-n transition. A rhodopsin molecule consists of an opsin protein molecule to which is attached an 11-ds-retinal molecule (Atlas E3 and 3). The latter resembles half a carotene molecule, showing Nature s economy in its use of available materials. The attachment is by the formation of a proton-ated Schiff s base, utilizing the CHO group of the chromophore and the terminal NHj group of the side chain of a lysine residue from opsin (5). The free 11-ds-retinal molecule absorbs in the ultraviolet, but attachment to the opsin protein molecule shifts the absorption into the visible region. The rhodopsin molecules are situated in the membranes of special cells (the rods and the cones ) that cover the retina. The opsin molecule is anchored into the cell membrane by two hydrophobic groups and largely surrounds the chromophore (Fig. 12.52). 

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