Phototaxis rhodopsin

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila [1]. Its chromophore is the deprotonated trans-p-coumaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore was shown to be the first overall step of the PYP photocycle, but the reaction path that leads to the formation of the cis isomer is not clear yet (for review see [2]). From time-resolved spectroscopy measurements on native PYP in solution, it came out that the excited-state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration [3-7]. On the other hand, chromophore H-bonding to the nearest amino acids was shown to play a key role in the trans excited state decay kinetics [3,8]. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, we made a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics were monitored by subpicosecond time-resolved absorption and gain spectroscopy. 

Sensory rhodopsin II (SRII, also called phobo-rhodopsin) is specialized for repellant phototaxis.5913 Blue light converts SRII487 in < 1 ms to UV-absorbing SRII360. It decays in 100 ms to SRn5/W) which reverts to the initial SRII487 in 0.5 s. The cycle is accompanied by swimming reversals that result in a repellent... 

Halobacteria contain four rhodopsins bacteriorhodopsin, halorhodopsin, sensor-yrhodopsin, and phoborhodopsin (Fig. 4.2A) [11-17]. Bacteriorhodopsin and halorhodopsin are light-driven ion pumps, which act as an outward proton pump and an inward Ch pump, respectively. Sensoryrhodopsin and phoborhodopsin are photoreceptors that act to produce attractant and repellent responses in phototaxis, respectively. These four archaeal rhodopsins have similar structures seven helices constitute the transmembrane portion of the protein, and a retinal chromophore is bound to a lysine residue of the seventh helix via a protonated SchifF base linkage (Fig. 4.1). A negatively charged counterion is present to stabilize the positive charge inside the protein the counterion is an aspartate except for in halorhodopsin, which possesses a chloride ion. In sensoryrhodopsin, interaction with a transmembrane transducer protein raises the pKa of the aspartate, so that the aspartate is protonated at neutral pH. 

Photochemical cis-trans isomerization is a major area of interest in modem photochemical research and is also studied as part of organic photochemistry. Photochemical cis-trans isomerization has a major role in many photobiological phenomena, such as vision (rhodopsin) [1], ATP synthesis (bacteriorhodopsin) [2], phototaxis (Chlamydomonas) [3], and other allied processes. It has practical application in industry [4-6], i.e., vitamin A and D processes. Furthermore, it is a likely candidate for many optoelectrical and optomechanical switching and storage devices [7]. In this chapter, mainly various aspects of cis-trans isomerization originating from the singlet excited state will be discussed. 

Sineshchekov and coworkers could show the direct involvement of both proteins in photoperception (50, 63). Action spectroscopy with knock-down mutants revealed a maximum sensitivity for ChRl at 500 and 470 nm for ChR2. Photophobic responses were impaired by reduction in ChR content while the involvement in phototaxis is still unclear. Chlamydomonas rhodopsins are light-gated ion channels. While the highest conductance is for H -ions the conductance for calcium plays the dominant role under physiological conditions. 

Rhodopsins are not the only photoreactive proteins. Another example is the photoactive yellow protein (PYP) (Figure 6.4), a small water-soluble photoreceptor responsible for the negative phototaxis of Halorhodospira halophila. This protein contains p-cou-maric acid (pCA) as a chromophore, which undergoes an ultrafast photoisomerization reaction immediately after light absorption. The photocycle involving different sequential intermediates, pG, pR, and pB, is then triggered by this photoreaction. 

Ridge, K.D. (2002). Algal rhodopsins Phototaxis receptors found at last. Curr. Biol 12, R588-R590. 

Spudich, E.N., Takahashi, T. and Spudich, J.L. (1989). Sensory rhodopsins I and n modulate a methylation/demethylation system in Hcdohacterium halobium phototaxis. Proc. Natl. Acad. Set. U.S.A. 86, 7746-7750. 

For both photosensor and photocoupling light quanta must be detected by specific receptor molecules. In blue-green algae and purple bacteria the same pigments, chlorophyll and bacteriochlorophyll respectively, serve as photoreceptors for photocoupling and phototaxis. Similarly bacterio-rhodopsin, the photosynthetic pigment of Halobacterium, also mediates a step-down photophobic response. Halobacterium also possesses a second... 

Biofuel industry is one of the upcoming industries who are in search of new and different strategies for the conversion of biomass into biofuel products. Since Archaea are exposed to strict sunlight, they have adapted a mechanism to convert solar energy by the transversion of protons and chloride ions for phototaxis. They harbor light sensitive proteins called the bacterial rhodopsin, a 25 kDa protein that carries the retinal group linked to lysine-216 by the Schififbase action. The bacteriorhodopsin protein is used in holography, spatial modulators, artificial retina and volumetric and associative optical memories (Alqueres et al., 2007). 

Sineshchekov, O.A., Jung, K.-H., and Spudich, J.L., Two rhodopsins mediate phototaxis to low-and high-intensity light in Chalmydomonas reinhardtii, PNAS, 99, 8689, 2002. 

There is also some evidence that a rhodopsin-hke molecule could be the photoreceptor pigment for phototaxis in Fabrea salina, as will be discussed below. 

The possibility that a rhodopsin might be the pigment for phototaxis in F. salina prompted us to use immunofluorescence to reveal the presence of a rhodopsin-like molecule on its plasma membrane. 

There are, however, other pieces of evidence that may indirectly suggest a role for a rhodopsin photoreceptor in Fabrea salina. In fact, preHminary experiments have shown that phototaxis is drastically reduced in the presence of hydroxylamine, which is known to interfere with the retinal-lysine binding in rhodopsins, and is strongly affected in the presence of zaprinast, a specific inhibitor of the phosphodiesterase known to be an essential component of the sensory transduction in vertebrate vision. In both cases, cell motflity is not affected by the presence of the drugs. 

In summary, the body of evidence currently available for F. salina indicates that these cells possess two different types of putative photoreceptors, a rhodopsin-like and a hypericin-hke pigment. The hypothesis that a rhodopsin pigment is responsible for phototaxis is supported by several pieces of evidence based on the similarity of its action spectrum to that of P. bursaria, by the effect of metabohc inhibitors such as hydroxylamine and zaprinast, by gene analysis, and by the suggested effect of Hght on membrane conductance. 

In the 1970s and early 1980s, four rhodopsins were discovered in the cytoplasmic membrane of the archaeon Halobacterium salinarum the light-driven ion pumps bacteriorhodopsin (BR ) and halor-hodopsin and the phototaxis receptors sensory rhodopsin 1 (SRP), and sensory rhodopsin 11... 

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