A Photo-Labile Thioether Linkage to Phycoviolobilin Provides the Foundation for the Blue/Green Photocycles in DXCF-Cyanobacteriochromes

The phytochrome superfamily encompasses a diverse collection of photochromic photoreceptors in plants and microorganisms that employ a covalently linked bilin cradled in a cGMP-phosphodiesterase/adenylyl-cyclase/FhlA (GAF) domain to detect light. Whereas most interconvert between red- and far-red-li...

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Bibliographic Details
Published in:Acta crystallographica. Section E, Structure reports online Structure reports online, 2013-01, Vol.21 (1), p.88-97
Main Authors: Burgie, E. Sethe, Walker, Joseph M., Phillips, George N., Vierstra, Richard D.
Format: Article
Language:English
Subjects:
Amino Acid Motifs
Bacterial Proteins - chemistry
Carbon
Crystallography, X-Ray
Cyanobacteria
Driving
Elongated structure
Foundations
Half-Life
Hydrogen Bonding
Kinetics
Linkages
Microorganisms
Models, Molecular
Perception
Photolysis
Photoreceptors
Photoreceptors, Microbial - chemistry
Photosynthesis
Phycobilins - chemistry
Protein Binding
Protein Structure, Tertiary
Structural Homology, Protein
Sulfides - chemistry
Thermodynamics
Thermosynechococcus elongatus
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Summary:The phytochrome superfamily encompasses a diverse collection of photochromic photoreceptors in plants and microorganisms that employ a covalently linked bilin cradled in a cGMP-phosphodiesterase/adenylyl-cyclase/FhlA (GAF) domain to detect light. Whereas most interconvert between red- and far-red-light-absorbing states, cyanobacteria also express variants called cyanobacteriochromes (CBCRs) that modify bilin absorption to collectively perceive the entire visible spectrum. Here, we present two X-ray crystallographic structures of the GAF domain from the blue/green photochromic CBCR PixJ from Thermosynechococcus elongatus. These structures confirm the hypothesis that CBCRs variably manipulate the chromophore π-conjugation system through isomerization and a second thioether linkage, in this case involving the bilin C10 carbon and Cys494 within a DXCF sequence characteristic of blue/green CBCRs. Biochemical studies support a mechanism for photoconversion whereby the second linkage ruptures on route to the green-light-absorbing state. Collectively, the TePixJ(GAF) models illustrate the remarkable structural and photochemical versatility among phytochromes and CBCRs in driving light perception. ▸ The Te-PixJ structure reveals the positions of the two DXCF-CBCR thioether linkages ▸ For blue/green photocycling, Te-PixJ utilizes a phycoviolobilin chromophore ▸ Rupture of the noncanonical DXCF thioether coincides with Pb/Pg photoconversion ▸ Te-PixJ bilin-binding and photochemistry diverge greatly from canonical phytochromes Phytochromes use a thioether-linked bilin to interconvert between red- and far-red light-absorbing states. Burgie et al. describe structures of PixJ, a cyanobacteriochrome, and how it enforces a blue/green photocycles by manipulating bilin π-conjugation through isomerization and a photo-labile thioether-linkage.
ISSN:0969-2126
1600-5368
1878-4186
1600-5368
DOI:10.1016/j.str.2012.11.001