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On the (un)coupling of the chromophore, tongue interactions, and overall conformation in a bacterial phytochrome

Journal article
Authors H. Takala
H. K. Lehtivuori
Oskar Berntsson
Ashley J Hughes
R. Nanekar
Stephan Niebling
Matthijs R Panman
Léocadie Henry
A. Menzel
Sebastian Westenhoff
J. A. Ihalainen
Published in Journal of Biological Chemistry
Volume 293
Issue 21
Pages 8161-8172
ISSN 0021-9258
Publication year 2018
Published at Department of Chemistry and Molecular Biology
Pages 8161-8172
Language en
Keywords photoreceptor, cell signaling, mutagenesis, protein structure, X-ray crystallography, structural biology, protein conformation, chromophore-binding domain, photoconversion, phytochrome, crystal-structure, deinococcus-radiodurans, plant phytochrome, binding, domain, bacteriophytochrome, photoconversion, module, agp1, ftir, rearrangements, Biochemistry & Molecular Biology
Subject categories Structural Biology


Phytochromes are photoreceptors in plants, fungi, and various microorganisms and cycle between metastable red light-absorbing (Pr) and far-red light-absorbing (Pfr) states. Their light responses are thought to follow a conserved structural mechanism that is triggered by isomerization of the chromophore. Downstream structural changes involve refolding of the so-called tongue extension of the phytochrome-specific GAF-related (PHY) domain of the photoreceptor. The tongue is connected to the chromophore by conserved DIP and PRXSF motifs and a conserved tyrosine, but the role of these residues in signal transduction is not clear. Here, we examine the tongue interactions and their interplay with the chromophore by substituting the conserved tyrosine (Tyr(263)) in the phytochrome from the extremophile bacterium Deinococcus radiodurans with phenylalanine. Using optical and FTIR spectroscopy, X-ray solution scattering, and crystallography of chromophore-binding domain (CBD) and CBD-PHY fragments, we show that the absence of the Tyr(263) hydroxyl destabilizes the -sheet conformation of the tongue. This allowed the phytochrome to adopt an -helical tongue conformation regardless of the chromophore state, hence distorting the activity state of the protein. Our crystal structures further revealed that water interactions are missing in the Y263F mutant, correlating with a decrease of the photoconversion yield and underpinning the functional role of Tyr(263) in phytochrome conformational changes. We propose a model in which isomerization of the chromophore, refolding of the tongue, and globular conformational changes are represented as weakly coupled equilibria. The results also suggest that the phytochromes have several redundant signaling routes.

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