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Characterization of two thermostable cyanobacterial phytochromes reveals global movements in the chromophore-binding domain during photoconversion
Citation key ISI:000257746100067
Author Ulijasz, Andrew T. and Cornilescu, Gabriel and von Stetten, David and Kaminski, Steve and Mroginski, Maria Andrea and Zhang, Junrui and Bhaya, Devaki and Hildebrandt, Peter and Vierstra, Richard D.
Pages 21251-21266
Year 2008
ISSN 0021-9258
DOI 10.1074/jbc.M801592200
Address 9650 ROCKVILLE PIKE, BETHESDA, MD 20814-3996 USA
Journal J. Biol. Chem.
Volume 283
Number 30
Month JUL 25
Abstract Photointerconversion between the red light-absorbing (Pr) form and the far-red light-absorbing (Pfr) form is the central feature that allows members of the phytochrome (Phy) superfamily to act as reversible switches in light perception. Whereas the chromophore structure and surrounding binding pocket of Pr have been described, those for Pfr have remained enigmatic for various technical reasons. Here we describe a novel pair of Phys from two thermophilic cyanobacteria, Synechococcus sp. OS-A and OS-B', that overcome several of these limitations. Like other cyanobacterial Phys, SyA-Cph1 and SyB-Cph1 covalently bind the bilin phycocyanobilin via their cGMP phosphodiesterase/ adenyl cyclase/FhlA (GAF) domains and then assume the photointerconvertible Pr and Pfr states with absorption maxima at 630 and 704 nm, respectively. However, they are naturally missing the N-terminal Per/Arndt/Sim domain common to others in the Phy superfamily. Importantly, truncations containing only the GAF domain are monomeric, photochromic, and remarkably thermostable. Resonance Raman and NMR spectroscopy show that all four pyrrole ring nitrogens of phycocyanobilin are protonated both as Pr and following red light irradiation, indicating that the GAF domain by itself can complete the Pr to Pfr photocycle. H-1-N-15 two-dimensional NMR spectra of isotopically labeled preparations of the SyB-Cph1 GAF domain revealed that a number of amino acids change their environment during photoconversion of Pr to Pfr, which can be reversed by subsequent photoconversion back to Pr. Through three-dimensional NMR spectroscopy before and after light photoexcitation, it should now be possible to define the movements of the chromophore and binding pocket during photoconversion. We also generated a series of strongly red fluorescent derivatives of SyB-Cph1, which based on their small size and thermostability may be useful as cell biological reporters.
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