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Publication Abstract from PubMed

Phytochromes are multidomain photoswitches that drive light perception in plants and microorganisms by coupling photoreversible isomerization of their bilin chromophore to various signaling cascades. How changes in bilin conformation affect output by these photoreceptors remains poorly resolved and might include several species-specific routes. Here, we present detailed three-dimensional models of the photosensing module (PSM), and a picture of an entire dimeric photoreceptor through structural analysis of the Deinococcus radiodurans phytochrome BphP assembled with biliverdin (BV). A 1.16 angstrom-resolution crystal structure of the bilin-binding pocket as Pr illuminated the intricate network of bilin/protein/water interactions and confirmed the protonation and ZZZssa conformation of BV. Structural and spectroscopic comparisons with the photochemically-compromised D207A mutant revealed that substitutions of Asp207 allow inclusion of cyclic porphyrins in addition to BV. A crystal structure of the entire PSM showed a head-to-head, twisted dimeric arrangement with bowed helical spines and a hairpin protrusion connecting the cGMP phosphodiesterase/adenylyl cyclase/FhlA (GAF) and phytochrome-specific (PHY) domains. A key conserved hairpin feature is its anti-parallel, two beta-strand stem, which we show by mutagenesis to be critical for BphP photochemistry. Single-particle electron microscopic images of the full-length BphP dimer in the dark-adapted Pr and photoactivated Pfr states revealed a large-scale reorientation of the PHY domain relative to the GAF domain, which alters the position of the downstream histidine kinase output module. Together, our data support a model whereby bilin photoisomerization alters GAF/PHY domain interactions through conformational modification of the hairpin, which regulates signaling by impacting the relationship between sister output modules.

Crystallographic and Electron Microscopic Analyses of a Bacterial Phytochrome Reveal Local and Global Rearrangements During Photoconversion.,Burgie ES, Wang T, Bussell AN, Walker JM, Li H, Vierstra RD J Biol Chem. 2014 Jul 8. pii: jbc.M114.571661. PMID:25006244^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.