From primary photochemistry to biological function in the blue-light photoreceptors PYP and AppA

Abstract

To properly respond to changes in fluency conditions, Nature has developed a variety of photosensors that modulate gene expression, enzyme activity and/or motility. Dedicated types have evolved, which can be classified in six families: rhodopsins, phytochromes, xanthopsins, cryptochromes, phototropins and BLUF-proteins. The photochemistry of the first three families is based on cis/trans isomerization of an ethylene bond. Surprisingly, the latter three all use flavin as their chromophore, but each with very different photochemistry. In this contribution we will discuss the molecular basis of signal generation in a xanthopsin (Photoactive Yellow Protein (PYP) from Halorhodospira halophila), a photoreceptor for negative phototaxis, and in a BLUF protein (AppA from Rhodobacter sphaeroides), a transcriptional anti-repressor. PYP is activated through trans/cis isomerization of the 7,8-vinyl bond of its 4-hydroxycinnamic acid chromophore. This initiates a photocycle with multiple intermediates, like pB, which is formed after intramolecular proton transfer. The negative charge thus formed in the interior of the protein triggers formation of a partially unfolded signaling state. For AppA much less is known about the underlying photochemistry. Available evidence suggests that it is based on a light-induced change in the hydrogen-bonding of its flavin chromophore and/or a change in hydrophobic stacking between the flavin and/or nearby aromatic amino acids like Y 21. A signaling state is formed within microseconds, which recovers with a rate of approximately 10(-3) s(-1). The change in conformation between receptor- and signaling-state in AppA, however, appear to be minute as compared to those in PYP. Here we review the underlying chemistry in the various steps of the photocycle of these two photoreceptor proteins and provide new data on their mechanism and function.

Redox properties of the Rhodobacter sphaeroides transcriptional regulatory proteins PpsR and AppA

“…In addition, AppA has an amino-terminal domain that contains a non-covalently bound FAD (Gomelsky and Klug 2002;. This FAD has been shown to be the chromophore that allows AppA to function as a blue-light photoreceptor (Anderson et al 2005;Dragnea et al 2005;van der Horst et al 2005). The interaction of AppA with PpsR is light regulated with light excited AppA unable to form an antirepressor-repressor co-complex .…”

“…While there is considerable information available about the effect of light absorption by the FAD binding domain of AppA (Anderson et al 2005;Dragnea et al 2005;van der Horst et al 2005) and on the interaction between PpsR and AppA (Bauer et al 2003), no information available about the oxidation-reduction chemistry of the many cysteine residues in AppA nor on the redox-active Cys in PpsR. For R. capsulatus CrtJ, E m values of -180 mV at pH 7.0 and of -245 mV at pH 8.0 have been measured for the redox-active Cys249/Cys420 disulfide/dithiol couple .…”

Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)

“…Note 2: Photochemical pathways have the advantage over thermal and catalytic methods of giving isomer mixtures (photostationary states) rich in thermodynamically unstable isomers. Note 3: Photoisomerization is the primary photochemical reaction of the chromophore in several biological photoreceptors such as retinal proteins (e.g., rhodopsin [133]), phytochromes [134], and the photoactive yellow protein [135].…”

In Vivo Sensitivity of Blue-Light-Dependent Signaling Mediated by AppA/PpsR or PrrB/PrrA in Rhodobacter sphaeroides

“…As shown before (10), light-dependent transcriptional control under anaerobic conditions is coordinated by an interplay of the AppA/PpsR and PrrB/PrrA regulatory systems. While AppA directly senses blue light through the FAD bound to the N-terminal BLUF domain (1,7,27), PrrB phosphorylation is regulated in an indirect manner via electron flow through the respiratory chain (13,14,22). To test the light sensitivity of the PrrB/PrrA-mediated response, puc expression was monitored in anaerobic cultures that were grown in a malate minimal salt medium in completely filled screw-capped flasks with 60 mM dimethyl sulfoxide added as a terminal electron acceptor.…”