A Hydrogen Bond Between Linear Tetrapyrrole and Conserved Aspartate Causes the Far-Red Shifted Absorption of Phytochrome Photoreceptors

Maximowitsch, Eglé; Domratcheva, Tatiana

doi:10.26434/chemrxiv.12278780

Cited by 2 publications

References 18 publications

(22 reference statements)

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Crystal structure of a far-red–sensing cyanobacteriochrome reveals an atypical bilin conformation and spectral tuning mechanism

Bandara

Rockwell

Zeng

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

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Cyanobacteriochromes (CBCRs) are small, linear tetrapyrrole (bilin)-binding photoreceptors in the phytochrome superfamily that regulate diverse light-mediated adaptive processes in cyanobacteria. More spectrally diverse than canonical red/far-red–sensing phytochromes, CBCRs were thought to be restricted to sensing visible and near UV light until recently when several subfamilies with far-red–sensing representatives (frCBCRs) were discovered. Two of these frCBCRs subfamilies have been shown to incorporate bilin precursors with larger pi-conjugated chromophores, while the third frCBCR subfamily uses the same phycocyanobilin precursor found in the bulk of the known CBCRs. To elucidate the molecular basis of far-red light perception by this third frCBCR subfamily, we determined the crystal structure of the far-red–absorbing dark state of one such frCBCR Anacy_2551g3 from Anabaena cylindrica PCC 7122 which exhibits a reversible far-red/orange photocycle. Determined by room temperature serial crystallography and cryocrystallography, the refined 2.7-Å structure reveals an unusual all-Z,syn configuration of the phycocyanobilin (PCB) chromophore that is considerably less extended than those of previously characterized red-light sensors in the phytochrome superfamily. Based on structural and spectroscopic comparisons with other bilin-binding proteins together with site-directed mutagenesis data, our studies reveal protein–chromophore interactions that are critical for the atypical bathochromic shift. Based on these analyses, we propose that far-red absorption in Anacy_2551g3 is the result of the additive effect of two distinct red-shift mechanisms involving cationic bilin lactim tautomers stabilized by a constrained all-Z,syn conformation and specific interactions with a highly conserved anionic residue.

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Crystal structure of a far-red–sensing cyanobacteriochrome reveals an atypical bilin conformation and spectral tuning mechanism

Bandara

Rockwell

Zeng

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

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Molecular Basis of Far-red Sensing in Cyanobacteriochrome

Bandara

Rockwell

Zeng

et al. 2020

Preprint

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Author contributionsXY designed the project; SB, SSM and XZ carried out cloning, purification, and crystallization; SB, XZ, SSM and NR conducted mutagenesis and spectroscopic experiments; SB, XZ, ZR, HS and XY collected monochromatic diffraction data; HS, CW and ZR collected Laue diffraction data; ZR processed the Laue data and carried out singlecrystal spectroscopic experiments; SB and XY determined, refined and analyzed crystal structures; NR carried out phylogenetic analysis; SB, XY, NR, MVM, and JCL interpreted the data; XY, NR, and JCL wrote the paper with inputs from all authors. Abstract (156 words)Cyanobacteriochromes are small, panchromatic photoreceptors in the phytochrome superfamily that regulate diverse light-mediated adaptive processes in cyanobacteria. The molecular basis of far-red (FR) light perception by cyanobacteriochromes is currently unknown. Here we report the crystal structure of a far-red-sensing cyanobacteriochrome from Anabaena cylindrica PCC 7122, which exhibits a reversible far-red/orange photocycle.The 2.7 Å structure of its FR-absorbing dark state, determined by room temperature serial crystallography and cryo-crystallography, reveals an all-Z,syn configuration of its bound linear tetrapyrrole (bilin) chromophore that is less extended than the bilin chromophores of all known phytochromes. Based on structural comparisons with other bilin-binding proteins and extensive spectral analyses on mutants, we identify key proteinchromophore interactions that enable far-red sensing in bilin-binding proteins. We propose that FR-CBCRs employ two distinct tuning mechanisms, which work together to produce a large batho-chromatic shift. Findings of this work have important implications for development and improvement of photoproteins with far-red absorption and fluorescence. Significance Statement (99 words)Phytochromes are well known far-red-light sensors found in plants that trigger adaptive responses to facilitate competition for light capture with neighboring plants. Red-and farred-sensing are critical to cyanobacteria living in the far-red-enriched shade of plants.Here we report the crystal structure of a far-red-sensing cyanobacteriochrome, a distant cyanobacterial relative of phytochrome. These studies shed insight into the poorly understood molecular basis of far-red-sensing by phytobilin-based photoreceptors.Owing to the deep tissue penetration of far-red light, far-red-sensing photoreceptors offer promising protein scaffolds for developing gene-based photoswitches, optoacoustic contrast agents and fluorescent probes for in situ imaging and optogenetic applications.

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A Hydrogen Bond Between Linear Tetrapyrrole and Conserved Aspartate Causes the Far-Red Shifted Absorption of Phytochrome Photoreceptors

Cited by 2 publications

References 18 publications

Crystal structure of a far-red–sensing cyanobacteriochrome reveals an atypical bilin conformation and spectral tuning mechanism

Crystal structure of a far-red–sensing cyanobacteriochrome reveals an atypical bilin conformation and spectral tuning mechanism

Molecular Basis of Far-red Sensing in Cyanobacteriochrome

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