Structure and photoreaction of photoactive yellow protein, a structural prototype of the PAS domain superfamily.

Photoactive yellow protein (PYP) is a water-soluble photosensor protein found in purple photosynthetic bacteria. Unlike bacterial rhodopsins, photosensor proteins composed of seven transmembrane helices and a retinal chromophore in halophilic archaebacteria, PYP is a highly soluble globular protein. The alpha/beta fold structure of PYP is a structural prototype of the PAS domain superfamily, many members of which function as sensors for various kinds of stimuli. To absorb a photon in the visible region, PYP has a p-coumaric acid chromophore binding to the cysteine residue via a thioester bond. It exists in a deprotonated trans form in the dark. The primary photochemical event is photo-isomerization of the chromophore from trans to cis form. The twisted cis chromophore in early intermediates is relaxed and finally protonated. Consequently, the chromophore becomes electrostatically neutral and rearrangement of the hydrogen-bonding network triggers overall structural change of the protein moiety, in which local conformational change around the chromophore is propagated to the N-terminal region. Thus, it is an ideal model for protein conformational changes that result in functional change, responding to stimuli and expressing physiological activity. In this paper, recent progress in investigation of the photoresponse of PYP is reviewed.