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Chromophore–Protein Interplay during the Phytochrome Photocycle Revealed by Step-Scan FTIR Spectroscopy
journal contribution
posted on 2018-09-05, 00:00 authored by Janne A. Ihalainen, Emil Gustavsson, Lea Schroeder, Serena Donnini, Heli Lehtivuori, Linnéa Isaksson, Christian Thöing, Vaibhav Modi, Oskar Berntsson, Brigitte Stucki-Buchli, Alli Liukkonen, Heikki Häkkänen, Elina Kalenius, Sebastian Westenhoff, Tilman KottkePhytochrome
proteins regulate many photoresponses of plants and
microorganisms. Light absorption causes isomerization of the biliverdin
chromophore, which triggers a series of structural changes to activate
the signaling domains of the protein. However, the structural changes
are elusive, and therefore the molecular mechanism of signal transduction
remains poorly understood. Here, we apply two-color step-scan infrared
spectroscopy to the bacteriophytochrome from Deinococcus radiodurans. We show by recordings in H2O and D2O that
the hydrogen bonds to the biliverdin D-ring carbonyl become disordered
in the first intermediate (Lumi-R) forming a dynamic microenvironment,
then completely detach in the second intermediate (Meta-R), and finally
reform in the signaling state (Pfr). The spectra reveal via isotope
labeling that the refolding of the conserved “PHY-tongue”
region occurs with the last transition between Meta-R and Pfr. Additional
changes in the protein backbone are detected already within microseconds
in Lumi-R. Aided by molecular dynamics simulations, we find that a
strictly conserved salt bridge between an arginine of the PHY tongue
and an aspartate of the chromophore binding domains is broken in Lumi-R
and the arginine is recruited to the D-ring CO. This rationalizes
how isomerization of the chromophore is linked to the global structural
rearrangement in the sensory receptor. Our findings advance the structural
understanding of phytochrome photoactivation.
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Lumi-RAdditional changesbiliverdin chromophoresignal transductionfindings advancelight absorption causes isomerizationDeinococcus radioduransprotein backboneD 2 OPfrH 2 Oargininebiliverdin D-ring carbonylStep-Scan FTIR Spectroscopy Phytochrome proteinschromophore binding domainsPhytochrome Photocycle Revealedhydrogen bondsMeta-Rphytochrome photoactivationPHY tonguedynamics simulationssalt bridge
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