Abstract
Phytochromes sense red light in plants and various microorganism. Light absorption causes structural changes within the protein, which alter its biochemical activity. Bacterial phytochromes are dimeric proteins, but the functional relevance of this arrangement remains unclear. Here, we use time-resolved X-ray scattering to reveal the solution structural change of a monomeric variant of the photosensory core module of the phytochrome from Deinococcus radiodurans. The data reveal two motions, a bend and a twist of the PHY domain with respect to the chromophore-binding domains. Infrared spectroscopy shows the refolding of the PHY tongue. We conclude that a monomer of the phytochrome photosensory core is sufficient to perform the light-induced structural changes. This implies that allosteric cooperation with the other monomer is not needed for structural activation. The dimeric arrangement may instead be intrinsic to the biochemical output domains of bacterial phytochromes.
Highlights
Prototypical phytochromes, the Pr state is the resting state and the Pfr state is metastable (Fig. 1(a))
We have previously shown how refolding of the PHY tongue in the Deinococcus radiodurans phytochrome is associated with the change in the relative orientation of the PHY domains
The turnover was estimated by titrating the laser power and measuring the amplitude of the difference scattering signal of the full-length dimeric phytochrome from D. radiodurans with details described in Ref. 21
Summary
An important additional chromophore-related interaction is with a “hairpin,” “tongue,” or “arm” extension of the PHY domain. Incident red/far-red light causes a Z-to-E isomerization of the C151⁄4C16 double bond in the chromophore.4–7 These changes are relayed and amplified to the rest of the protein. The tongue adopts a b-turn-like conformation in the Pr state but an a-helix and coil in the Pfr state.8,10–12 Thereby, it switches between two highly conserved interaction patterns that include the PRxSF motif of the tongue and the DIP motif in the GAF domain. The sister PHY domains of the dimeric photosensory module (PAS-GAF-PHY) separate by several nanometers in the Pfr state, compared with the Pr state.. We study the light-induced conformational changes of the monomeric variant of the photosensory module from D. radiodurans (PAS-GAF-PHYmon).. We discuss the results in terms of potential allostery and the role of the dimeric arrangement of phytochromes
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