Phototaxis photoactive yellow proteins

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. 

The Photoactive Yellow Protein (PYP) is thought to be the photoreceptor responsible for the negative phototaxis of the bacterium Halorhodospira halophila [1]. Its chromophore, the deprotonated 4-hydroxycinnamic (or p-coumaric) acid, is covalently linked to the side chain of the Cys69 residue by a thioester bond. Trans-cis photoisomerization of the chromophore was proved to occur during the early steps of the PYP photocycle. Nevertheless, the reaction pathway leading to the cis isomer is still discussed (for a review, see ref. [2]). Time-resolved spectroscopy showed that it involves subpicosecond and picosecond components [3-7], some of which could correspond to a flipping motion of the chromophore carbonyl group [8,9]. 

Photoactive yellow protein (PYP) was discovered 20 year ago in Halorhodospira halophila, then known as Ectothiorhodospira halophila [1,2]. In several halophilic purple bacteria it has a vital role in the avoidance response to blue light (phototaxis). It has been thoroughly studied as a model photoreceptor system and as the structural prototype for the PAS class of signal transduction proteins. PYP has 125 amino acid residues in an a// -fold with six antiparallel /1-sheets and several helices (see Fig. 5.1). The covalently bound p-coumaric acid chromophore is linked to the only cysteine in the protein (Cys69) (see Fig. 5.1). Hellingwerf has published an excellent review of the photophysical behavior of PYP [1],... 

Fig. 1. Summary of avaiiabie knowiedge on the phototaxis signaiing pathways in H. sallnarum, R. sphaeroides, and H. halophila in a Che-iike reaction scheme. H. sali-narum contains the photoreceptors SRi and SRii, which are compiexed in the membrane to their signal transducers Htri and Htrii. These transducers modulate the autokinase activity of CheA and thus modulate the phosphorylation status of CheY. Phototaxis of R. sphaeroides proceeds via its photosynthetic reaction center (RC) and electron transfer chain (ETC) via a putative redox sensor. Positive phototaxis in H. halophila occurs via a similar pathway, while its negative phototaxis is triggered by photoactive yellow protein (PYP). The signal transduction pathway for PYP is unknown one candidate is the Che system. Possible adaptation mechanisms have been omitted from this figure.

The positive phototaxis response is triggered through the photosynthetic machinery, as is the case for Rb. sphaeroides. The wavelength dependence of the negative phototaxis response has indicated that photoactive yellow protein (PYP) functions as the dedicated photoreceptor for this response. While the mechanism for the light activation of purified PYP has been unraveled in... 

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. 

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