1
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Beck WF. Intramolecular charge transfer and the function of vibronic excitons in photosynthetic light harvesting. PHOTOSYNTHESIS RESEARCH 2024:10.1007/s11120-024-01095-5. [PMID: 38656684 DOI: 10.1007/s11120-024-01095-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/26/2023] [Accepted: 03/15/2024] [Indexed: 04/26/2024]
Abstract
A widely discussed explanation for the prevalence of pairs or clusters of closely spaced electronic chromophores in photosynthetic light-harvesting proteins is the presence of ultrafast and highly directional excitation energy transfer pathways mediated by vibronic excitons, the delocalized optical excitations derived from mixing of the electronic and vibrational states of the chromophores. We discuss herein the hypothesis that internal conversion processes between exciton states on the <100 fs timescale are possible when the excitonic potential energy surfaces are controlled by the vibrational modes that induce charge transfer character in a strongly coupled system of chromophores. We discuss two examples, the peridinin-chlorophyll protein from marine dinoflagellates and the intact phycobilisome from cyanobacteria, in which the intramolecular charge-transfer (ICT) character arising from out-of-plane distortion of the conjugation of carotenoid or bilin chromophores also results in localization of the initially delocalized optical excitation on the vibrational timescale. Tuning of the ground state conformations of the chromophores to manipulate their ICT character provides a natural photoregulatory mechanism, which would control the overall quantum yield of excitation energy transfer by turning on and off the delocalized character of the optical excitations.
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Affiliation(s)
- Warren F Beck
- Department of Chemistry, Michigan State University, East Lansing, MI, 48824, USA.
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2
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Rockwell NC, Lagarias JC. Cyanobacteriochromes from Gloeobacterales Provide New Insight into the Diversification of Cyanobacterial Photoreceptors. J Mol Biol 2024; 436:168313. [PMID: 37839679 DOI: 10.1016/j.jmb.2023.168313] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2023] [Revised: 09/15/2023] [Accepted: 10/10/2023] [Indexed: 10/17/2023]
Abstract
The phytochrome superfamily comprises three groups of photoreceptors sharing a conserved GAF (cGMP-specific phosphodiesterases, cyanobacterial adenylate cyclases, and formate hydrogen lyase transcription activator FhlA) domain that uses a covalently attached linear tetrapyrrole (bilin) chromophore to sense light. Knotted red/far-red phytochromes are widespread in both bacteria and eukaryotes, but cyanobacteria also contain knotless red/far-red phytochromes and cyanobacteriochromes (CBCRs). Unlike typical phytochromes, CBCRs require only the GAF domain for bilin binding, chromophore ligation, and full, reversible photoconversion. CBCRs can sense a wide range of wavelengths (ca. 330-750 nm) and can regulate phototaxis, second messenger metabolism, and optimization of the cyanobacterial light-harvesting apparatus. However, the origins of CBCRs are not well understood: we do not know when or why CBCRs evolved, or what selective advantages led to retention of early CBCRs in cyanobacterial genomes. In the current work, we use the increasing availability of genomes and metagenome-assembled-genomes from early-branching cyanobacteria to explore the origins of CBCRs. We reaffirm the earliest branches in CBCR evolution. We also show that early-branching cyanobacteria contain late-branching CBCRs, implicating early appearance of CBCRs during cyanobacterial evolution. Moreover, we show that early-branching CBCRs behave as integrators of light and pH, providing a potential unique function for early CBCRs that led to their retention and subsequent diversification. Our results thus provide new insight into the origins of these diverse cyanobacterial photoreceptors.
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Affiliation(s)
- Nathan C Rockwell
- 31 Briggs Hall, Department of Molecular and Cell Biology, One Shields Avenue, University of California at Davis, Davis, CA 95616, USA.
| | - J Clark Lagarias
- 31 Briggs Hall, Department of Molecular and Cell Biology, One Shields Avenue, University of California at Davis, Davis, CA 95616, USA.
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3
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Kannan P, Oh J, Yeon YJ, Park YI, Seo MH, Park K. Computational identification of key residues regulating fluorescence emission in a red/green cyanobacteriochrome. Proteins 2024; 92:106-116. [PMID: 37646483 DOI: 10.1002/prot.26586] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2023] [Revised: 08/03/2023] [Accepted: 08/16/2023] [Indexed: 09/01/2023]
Abstract
Cyanobacteriochromes (CBCRs) are linear tetrapyrrole bilin-binding photoreceptors of cyanobacteria that exhibit high spectral diversity, gaining attention in optogenetics and bioimaging applications. Several engineering studies on CBCRs were attempted, especially for designing near-infrared (NIR) fluorescent proteins with longer fluorescence wavelengths. However, despite continuous efforts, a key component regulating fluorescence emission property in CBCRs is still poorly understood. As a model system, we focused on red/green CBCR Slr1393g3, from the unicellular cyanobacterium Synechocystis sp. PCC 6803 to engineer Pr to get far-red light-emitting property. Energy profiling and pairwise structural comparison of Slr1393g3 variants effectively reveal the mutations that are critical to the fluorescence changes. H497 seems to play a key role in stabilizing the chromophore environment, especially the α3 helix, while H495, T499, and Q502 are potential key residues determining fluorescence emission peak wavelength. We also found that mutations of α2 and α4 helical regions are closely related to the chromophore binding stability and likely affect fluorescence properties. Taken together, our computational analysis suggests that the fluorescence of Slr1393g3 is mainly controlled by the stabilization of the chromophore binding pocket. The predicted key residues potentially regulating the fluorescence emission property of a red/green CBCR will be advantageous for designing improved NIR fluorescent protein when combined with in vitro molecular evolution approaches.
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Affiliation(s)
- Priyadharshini Kannan
- Natural Product Informatics Research Center, Korea Institute of Science and Technology, Gangneung, Republic of Korea
- Department of Biochemical Engineering, Gangneung-Wonju National University, Gangneung, Republic of Korea
| | - Jisung Oh
- Department of Biochemical Engineering, Gangneung-Wonju National University, Gangneung, Republic of Korea
- Natural Product Research Center, Korea Institute of Science and Technology, Gangneung, Republic of Korea
| | - Young Joo Yeon
- Department of Biochemical Engineering, Gangneung-Wonju National University, Gangneung, Republic of Korea
| | - Youn-Il Park
- Department of Biological Sciences, Chungnam National University, Daejeon, Republic of Korea
| | - Moon-Hyeong Seo
- Natural Product Research Center, Korea Institute of Science and Technology, Gangneung, Republic of Korea
- Department of Convergence Medicine, Wonju Severance Christian Hospital, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea
- Department of YM-KIST Bio-Health Convergence, Yonsei University, Wonju, Republic of Korea
| | - Keunwan Park
- Natural Product Informatics Research Center, Korea Institute of Science and Technology, Gangneung, Republic of Korea
- Department of YM-KIST Bio-Health Convergence, Yonsei University, Wonju, Republic of Korea
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4
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Henning RW, Kosheleva I, Šrajer V, Kim IS, Zoellner E, Ranganathan R. BioCARS: Synchrotron facility for probing structural dynamics of biological macromolecules. STRUCTURAL DYNAMICS (MELVILLE, N.Y.) 2024; 11:014301. [PMID: 38304444 PMCID: PMC10834067 DOI: 10.1063/4.0000238] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/18/2023] [Accepted: 01/10/2024] [Indexed: 02/03/2024]
Abstract
A major goal in biomedical science is to move beyond static images of proteins and other biological macromolecules to the internal dynamics underlying their function. This level of study is necessary to understand how these molecules work and to engineer new functions and modulators of function. Stemming from a visionary commitment to this problem by Keith Moffat decades ago, a community of structural biologists has now enabled a set of x-ray scattering technologies for observing intramolecular dynamics in biological macromolecules at atomic resolution and over the broad range of timescales over which motions are functionally relevant. Many of these techniques are provided by BioCARS, a cutting-edge synchrotron radiation facility built under Moffat leadership and located at the Advanced Photon Source at Argonne National Laboratory. BioCARS enables experimental studies of molecular dynamics with time resolutions spanning from 100 ps to seconds and provides both time-resolved x-ray crystallography and small- and wide-angle x-ray scattering. Structural changes can be initiated by several methods-UV/Vis pumping with tunable picosecond and nanosecond laser pulses, substrate diffusion, and global perturbations, such as electric field and temperature jumps. Studies of dynamics typically involve subtle perturbations to molecular structures, requiring specialized computational techniques for data processing and interpretation. In this review, we present the challenges in experimental macromolecular dynamics and describe the current state of experimental capabilities at this facility. As Moffat imagined years ago, BioCARS is now positioned to catalyze the scientific community to make fundamental advances in understanding proteins and other complex biological macromolecules.
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Affiliation(s)
- Robert W. Henning
- BioCARS, Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, USA
| | - Irina Kosheleva
- BioCARS, Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, USA
| | - Vukica Šrajer
- BioCARS, Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, USA
| | - In-Sik Kim
- BioCARS, Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, USA
| | - Eric Zoellner
- BioCARS, Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, USA
| | - Rama Ranganathan
- BioCARS, Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, USA
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Hoshino H, Narikawa R. Novel cyanobacteriochrome photoreceptor with the second Cys residue showing atypical orange/blue reversible photoconversion. Photochem Photobiol Sci 2023; 22:251-261. [PMID: 36156209 DOI: 10.1007/s43630-022-00310-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2022] [Accepted: 09/17/2022] [Indexed: 11/30/2022]
Abstract
Cyanobacteriochromes (CBCRs) are cyanobacterial linear tetrapyrrole-binding photoreceptors distantly related to phytochromes. Only the GAF domain is needed for chromophore incorporation and proper photoconversion of the CBCRs. Most CBCR GAF domains possess the canonical Cys residue stably ligating to the chromophore. DXCF-type CBCR GAF domains also possess a second Cys residue within the DXCF motif. This second Cys residue reversibly ligates to the C10 of the chromophore. The Cys adduct formation is mostly observed for the dark-adapted state but not for the photoproduct state. In this study, we discovered novel CBCR GAF domains with a DXCI motif instead of the DXCF motif. Since these CBCR GAF domains are categorized into two subfamilies (DXCI-1 and DXCI-2), the GAF domains from each subfamily were analyzed. Although the CBCR GAF domain belonging to the DXCI-2 subfamily showed orange/green reversible photoconversion without transient Cys ligation, the CBCR GAF domain belonging to the DXCI-1 subfamily showed reversible photoconversion between an orange-absorbing dark-adapted state and a blue-absorbing photoproduct state. This indicates that the second Cys residue is covalently bound to the C10 of the chromophore in the photoproduct state but not in the dark-adapted state. Since the covalent bond formation in the photoproduct state is atypical, site-directed mutagenesis was conducted to understand the molecular mechanism of this GAF domain. The Ile residue within the DXCI motif may be key for covalent bond formation in the photoproduct state.
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Affiliation(s)
- Hiroki Hoshino
- Department of Biological Sciences, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo, 192-0397, Japan
| | - Rei Narikawa
- Department of Biological Sciences, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo, 192-0397, Japan.
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6
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Meng X, Liu L, Chen X. Bacterial photosynthesis: state-of-the-art in light-driven carbon fixation in engineered bacteria. Curr Opin Microbiol 2022; 69:102174. [DOI: 10.1016/j.mib.2022.102174] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2022] [Revised: 05/24/2022] [Accepted: 05/30/2022] [Indexed: 11/03/2022]
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Light- and pH-dependent structural changes in cyanobacteriochrome AnPixJg2. Photochem Photobiol Sci 2022; 21:447-469. [PMID: 35394641 DOI: 10.1007/s43630-022-00204-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2021] [Accepted: 03/03/2022] [Indexed: 10/18/2022]
Abstract
Cyanobacteriochromes (CBCRs) are phytochrome-related photosensory proteins that play an essential role in regulating phototaxis, chromatic acclimation, and cell aggregation in cyanobacteria. Here, we apply solid-state NMR spectroscopy to the red/green GAF2 domain of the CBCR AnPixJ assembled in vitro with a uniformly 13C- and 15N-labeled bilin chromophore, tracking changes in electronic structure, geometry, and structural heterogeneity of the chromophore as well as intimate contacts between the chromophore and protein residues in the photocycle. Our data confirm that the bilin ring D is strongly twisted with respect to the B-C plane in both dark and photoproduct states. We also identify a greater structural heterogeneity of the bilin chromophore in the photoproduct than in the dark state. In addition, the binding pocket is more hydrated in the photoproduct. Observation of interfacial 1H contacts of the photoproduct chromophore, together with quantum mechanics/molecular mechanics (QM/MM)-based structural models for this photoproduct, clearly suggests the presence of a biprotonated (cationic) imidazolium side-chain for a conserved histidine residue (322) at a distance of ~2.7 Å, generalizing the recent theoretical findings that explicitly link the structural heterogeneity of the dark-state chromophore to the protonation of this specific residue. Moreover, we examine pH effects on this in vitro assembled holoprotein, showing a substantially altered electronic structure and protonation of the photoproduct chromophore even with a small pH drop from 7.8 to 7.2. Our studies provide further information regarding the light- and pH-induced changes of the chromophore and the rearrangements of the hydrogen-bonding and electrostatic interaction network around it. Possible correlations between structural heterogeneity of the chromophore, protonation of the histidine residue nearby, and hydration of the pocket in both photostates are discussed.
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Rockwell NC, Moreno MV, Martin SS, Lagarias JC. Protein-chromophore interactions controlling photoisomerization in red/green cyanobacteriochromes. Photochem Photobiol Sci 2022; 21:471-491. [PMID: 35411484 PMCID: PMC9609751 DOI: 10.1007/s43630-022-00213-3] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2022] [Accepted: 03/21/2022] [Indexed: 10/18/2022]
Abstract
Photoreceptors in the phytochrome superfamily use 15,16-photoisomerization of a linear tetrapyrrole (bilin) chromophore to photoconvert between two states with distinct spectral and biochemical properties. Canonical phytochromes include master regulators of plant growth and development in which light signals trigger interconversion between a red-absorbing 15Z dark-adapted state and a metastable, far-red-absorbing 15E photoproduct state. Distantly related cyanobacteriochromes (CBCRs) carry out a diverse range of photoregulatory functions in cyanobacteria and exhibit considerable spectral diversity. One widespread CBCR subfamily typically exhibits a red-absorbing 15Z dark-adapted state similar to that of phytochrome that gives rise to a distinct green-absorbing 15E photoproduct. This red/green CBCR subfamily also includes red-inactive examples that fail to undergo photoconversion, providing an opportunity to study protein-chromophore interactions that either promote photoisomerization or block it. In this work, we identified a conserved lineage of red-inactive CBCRs. This enabled us to identify three substitutions sufficient to block photoisomerization in photoactive red/green CBCRs. The resulting red-inactive variants faithfully replicated the fluorescence and circular dichroism properties of naturally occurring examples. Converse substitutions restored photoconversion in naturally red-inactive CBCRs. This work thus identifies protein-chromophore interactions that control the fate of the excited-state population in red/green cyanobacteriochromes.
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Affiliation(s)
- Nathan C Rockwell
- Department of Molecular and Cellular Biology, University of California at Davis, Davis, CA, 95616, USA.
| | - Marcus V Moreno
- Department of Molecular and Cellular Biology, University of California at Davis, Davis, CA, 95616, USA
| | - Shelley S Martin
- Department of Molecular and Cellular Biology, University of California at Davis, Davis, CA, 95616, USA
| | - J Clark Lagarias
- Department of Molecular and Cellular Biology, University of California at Davis, Davis, CA, 95616, USA.
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Okuda Y, Miyoshi R, Kamo T, Fujisawa T, Nagae T, Mishima M, Eki T, Hirose Y, Unno M. Raman Spectroscopy of an Atypical C15-E,syn Bilin Chromophore in Cyanobacteriochrome RcaE. J Phys Chem B 2022; 126:813-821. [DOI: 10.1021/acs.jpcb.1c09652] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Yuji Okuda
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Risako Miyoshi
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Takanari Kamo
- Department of Applied Chemistry and Life Science, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan
| | - Tomotsumi Fujisawa
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Takayuki Nagae
- Synchrotron Radiation Research Center, Nagoya University, Chikusa, Nagoya 464-8603, Japan
| | - Masaki Mishima
- Department of Molecular Biophysics, School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, Hachioji, Tokyo 192-0392, Japan
| | - Toshihiko Eki
- Department of Applied Chemistry and Life Science, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan
| | - Yuu Hirose
- Department of Applied Chemistry and Life Science, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan
| | - Masashi Unno
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
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Lehtinen K, Nokia MS, Takala H. Red Light Optogenetics in Neuroscience. Front Cell Neurosci 2022; 15:778900. [PMID: 35046775 PMCID: PMC8761848 DOI: 10.3389/fncel.2021.778900] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2021] [Accepted: 12/02/2021] [Indexed: 12/25/2022] Open
Abstract
Optogenetics, a field concentrating on controlling cellular functions by means of light-activated proteins, has shown tremendous potential in neuroscience. It possesses superior spatiotemporal resolution compared to the surgical, electrical, and pharmacological methods traditionally used in studying brain function. A multitude of optogenetic tools for neuroscience have been created that, for example, enable the control of action potential generation via light-activated ion channels. Other optogenetic proteins have been used in the brain, for example, to control long-term potentiation or to ablate specific subtypes of neurons. In in vivo applications, however, the majority of optogenetic tools are operated with blue, green, or yellow light, which all have limited penetration in biological tissues compared to red light and especially infrared light. This difference is significant, especially considering the size of the rodent brain, a major research model in neuroscience. Our review will focus on the utilization of red light-operated optogenetic tools in neuroscience. We first outline the advantages of red light for in vivo studies. Then we provide a brief overview of the red light-activated optogenetic proteins and systems with a focus on new developments in the field. Finally, we will highlight different tools and applications, which further facilitate the use of red light optogenetics in neuroscience.
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Affiliation(s)
- Kimmo Lehtinen
- Department of Biological and Environmental Science, Nanoscience Center, University of Jyväskylä, Jyväskylä, Finland
| | - Miriam S. Nokia
- Department of Psychology, University of Jyväskylä, Jyväskylä, Finland
- Centre for Interdisciplinary Brain Research, University of Jyväskylä, Jyväskylä, Finland
| | - Heikki Takala
- Department of Biological and Environmental Science, Nanoscience Center, University of Jyväskylä, Jyväskylä, Finland
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Hu JH, Chang JW, Xu T, Wang J, Wang X, Lin R, Duanmu D, Liu L. Structural basis of bilin binding by the chlorophyll biosynthesis regulator GUN4. Protein Sci 2021; 30:2083-2091. [PMID: 34382282 DOI: 10.1002/pro.4164] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2021] [Revised: 08/05/2021] [Accepted: 08/09/2021] [Indexed: 11/05/2022]
Abstract
The chlorophyll biosynthesis regulator GENOMES UNCOUPLED 4 (GUN4) is conserved in nearly all oxygenic photosynthetic organisms. Recently, GUN4 has been found to be able to bind the linear tetrapyrroles (bilins) and stimulate the magnesium chelatase activity in the unicellular green alga Chlamydomonas reinhardtii. Here, we characterize GUN4 proteins from Arabidopsis thaliana and the cyanobacterium Synechocystis sp. PCC 6803 for their ability to bind bilins, and present the crystal structures of Synechocystis GUN4 in biliverdin-bound, phycocyanobilin-bound, and phytochromobilin-bound forms at the resolutions of 1.05, 1.10, and 1.70 Å, respectively. These linear molecules adopt a cyclic-helical conformation, and bind more tightly than planar porphyrins to the tetrapyrrole-binding pocket of GUN4. Based on structural comparison, we propose a working model of GUN4 in regulation of tetrapyrrole biosynthetic pathway, and address the role of the bilin-bound GUN4 in retrograde signaling.
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Affiliation(s)
- Jiu-Hui Hu
- School of Life Sciences, Anhui University, Hefei, China
| | | | - Tao Xu
- School of Life Sciences, Anhui University, Hefei, China
| | - Jia Wang
- School of Life Sciences, Anhui University, Hefei, China.,Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing, China
| | - Xiao Wang
- School of Life Sciences, Anhui University, Hefei, China.,Anhui Key Laboratory of Modern Biomanufacturing, Anhui University, Hefei, China
| | - Rongcheng Lin
- Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing, China
| | - Deqiang Duanmu
- State Key Laboratory of Agricultural Microbiology, College of Life Science and Technology, Huazhong Agricultural University, Wuhan, China
| | - Lin Liu
- School of Life Sciences, Anhui University, Hefei, China.,Anhui Key Laboratory of Modern Biomanufacturing, Anhui University, Hefei, China
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Structural basis of the protochromic green/red photocycle of the chromatic acclimation sensor RcaE. Proc Natl Acad Sci U S A 2021; 118:2024583118. [PMID: 33972439 DOI: 10.1073/pnas.2024583118] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cyanobacteriochromes (CBCRs) are bilin-binding photosensors of the phytochrome superfamily that show remarkable spectral diversity. The green/red CBCR subfamily is important for regulating chromatic acclimation of photosynthetic antenna in cyanobacteria and is applied for optogenetic control of gene expression in synthetic biology. It is suggested that the absorption change of this subfamily is caused by the bilin C15-Z/C15-E photoisomerization and a subsequent change in the bilin protonation state. However, structural information and direct evidence of the bilin protonation state are lacking. Here, we report a high-resolution (1.63Å) crystal structure of the bilin-binding domain of the chromatic acclimation sensor RcaE in the red-absorbing photoproduct state. The bilin is buried within a "bucket" consisting of hydrophobic residues, in which the bilin configuration/conformation is C5-Z,syn/C10-Z,syn/C15-E,syn with the A- through C-rings coplanar and the D-ring tilted. Three pyrrole nitrogens of the A- through C-rings are covered in the α-face with a hydrophobic lid of Leu249 influencing the bilin pK a, whereas they are directly hydrogen bonded in the β-face with the carboxyl group of Glu217. Glu217 is further connected to a cluster of waters forming a hole in the bucket, which are in exchange with solvent waters in molecular dynamics simulation. We propose that the "leaky bucket" structure functions as a proton exit/influx pathway upon photoconversion. NMR analysis demonstrated that the four pyrrole nitrogen atoms are indeed fully protonated in the red-absorbing state, but one of them, most likely the B-ring nitrogen, is deprotonated in the green-absorbing state. These findings deepen our understanding of the diverse spectral tuning mechanisms present in CBCRs.
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