Rhodopsin from halobacteria

Contrasting Functions of Rhodopsin and the Retinaldehyde Proteins from Halobacteria... 

Both in the case of sensory rhodopsin in humans and of bacteriorhodopsin (a heptahelical membrane protein in halobacteria which is not coupled to a G protein) translocation of a Schiff-base proton is the essential step in making the protein functional (reviewed in ref 58). In rhodopsin the conversion of the inactive AH state to the AHI state that binds to the G protein is coupled to proton transfer from the Schiff base to the counterion, Glul 13, and proton uptake from the medium to the highly conserved Glul34, which serves as proton acceptor. Based on that similarity, one could consider sensory rhodopsin as an incomplete proton pump. Furthermore, a property shared by all G-protein-coupled receptors is a triplet, formed by residues 134-136 in rhodopsin, consisting of Glu-Arg-Tyr. The consequences of mutational replacement of Glul34 supports the notion that the state of protonation of this amino add is crudal for activity, and that its protonation triggers the conformational transition of the receptor from the inactive to the active state. 

The purple membrane is a unique patch of membrane in halobacteria [118-120] it contains a single protein of 26000 molecular weight and the protein is 75% of the mass of the membrane. The remainder is primarily phospholipid. The protein is called bacteriorhodopsin, in analogy to rhodopsin. Characterization by electron microscopy has shown the protein to be comprised of seven helical rods traversing back and forth across the membrane [121]. This is interpreted to indicate some 70-75% a-helix, the presence of which is also indicated from X-ray diffraction data [122,123]. Accordingly, the purple membrane provides an especially interesting opportunity to apply the preceding analyses for the correction of distortions in the ellipticity patterns of biomembranes. 

Even though rhodopsin and bacteriorhodopsin appear to differ fundamentally in their function, mode of action, structure, and photochemistry, it has been suggested (Stoeckenius et ai, 1979) that the two proteins did not evolve independently. One possibility is that the halobacteria may have acquired bacteriorhodopsin by gene transfer from a eukaryote (Stoeckenius et al., 1979). However, a preliminary report by Hargrave et al. (1983) claims that no statistically significant sequence homology can be found. Thus, the resemblance between these proteins appears to be only superficial. 

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