Proteorhodopsin

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Proteorhodopsin is a photoactive retinylidene protein in marine bacterioplanktons. Just like the homologous pigment bacteriorhodopsin found in some archaea, it consists of a transmembrane protein bound to a retinal molecule and functions as a light-driven proton pump. Some members of the family (of more than 800 types) are believed to have sensory functions.

Proteorhodopsin was first discovered in 2000. It was found in the genomes of several species of uncultivated marine γ-proteobacteria present in the Eastern Pacific Ocean, Central North Pacific Ocean and Southern Ocean, Antarctica. Subsequently, genes of proteorhodopsin variants have been identified in samples from the Mediterranean and Red Seas and the Sargasso Sea and the Sea of Japan.

On comparison to its better-known archaeal homolog bacteriorhodopsin, most of the active site residues of known importance to the bacteriorhodopsin mechanism are conserved in proteorhodopsin. Homologues of the active site residues Arg82, Asp85 (the primary proton acceptor), Asp212 and Lys216 (the retinal Schiff base binding site) in bacteriorhodopsin are conserved as Arg94, Asp97, Asp227 and Lys231 in proteorhodopsin. However, in proteorhodopsin, there are no carboxylic acid residues directly homologous to Glu194 or Glu204 of bacteriorhodopsin, which are thought to be involved in the proton release pathway at the extracellular surface.

It seems likely that proteorhodopsin functions throughout the Earth's oceans as a light-driven H+ pump, by a mechanism similar to that of bacteriorhodopsin. As in bacteriorhodopsin, the retinal chromophore of bacteriorhodopsin is covalently bound to the apoprotein via a protonated Schiff base at Lys231. The configuration of the retinal chromophore in unphotolyzed proteorhodopsin is predominantly all-trans, and changes to 13-cis upon illumination with light. Several models of the complete proteorhodopsin photocycle have been proposed, based on FTIR and UV–visible spectroscopy; they resemble established photocycle models for bacteriorhodopsin.

If the gene for proteorhodopsin is inserted into E. coli and retinal is given to these modified bacteria, then they will incorporate the pigment into their cell membrane and will pump protons in the presence of light.

[edit] References

1. Beja O, Aravind L, Koonin EV, Suzuki MT, Hadd A, Nguyen LP, Jovanovich SB, Gates CM, Feldman RA, Spudich JL, Spudich EN, DeLong EF. "Bacterial rhodopsin: Evidence for a new type of phototrophy in the sea." Science 289:1902-1906. (2000) Entrez PubMed 10988064
2. O. Béjà, E.N. Spudich, J.L. Spudich, M. Leclerc and E.F. DeLong, Proteorhodopsin phototrophy in the ocean, Nature 411 (2001), pp. 786–789. Entrez PubMed 11459054
3. R.A. Krebs, U. Alexiev, R. Partha, A.M. DeVita and M.S. Braiman, Detection of fast light-activated H+ release and M intermediate formation from proteorhodopsin, BMC Physiol. 2 (2002), p. 5. Entrez PubMed 11943070
4. B.R. Kelemen, M. Du and R.B. Jensen, Proteorhodopsin in living color: diversity of spectral properties within living bacterial cells, Biochim. Biophys. Acta 1618 (2003), pp. 25–32. Entrez PubMed 14643930
5. J.C. Venter, K. Remington, J.F. Heidelberg, A.L. Halpern, D. Rusch, J.A. Eisen, D. Wu, I. Paulsen, K.E. Nelson, W. Nelson, D.E. Fouts, S. Levy, A.H. Knap, M.W. Lomas, K. Nealson, O. White, J. Peterson, J. Hoffman, R. Parsons, H. Baden-Tillson, C. Pfannkoch, Y.H. Rogers and H.O. Smith, Environmental genome shotgun sequencing of the Sargasso Sea, Science 304 (2004), pp. 66–74. Entrez PubMed 15001713
6. Xiao, Y.; Partha, R., Krebs, R.; Braiman, M. S. Time-Resolved FTIR Spectroscopy of the Photointermediates Involved in Fast Transient H+ Release by Proteorhodopsin. J. Phys. Chem. B, 2005, 109(1), 634-641
7. Partha, R; Krebs, R.; Caterino, T. L.; Braiman, M. S. Weakened coupling of conserved arginine to the proteorhodopsin chromophore and its counterion implies structural differences from bacteriorhodopsin. Bioch. Biophys. Acta. 2005 1708: 1. 6-12 Entrez PubMed 15949979