Scientists collaborate to achieve landmark research results in the field of photosynthesis

Tsinghua News Network, March 16 — Phycobilisomes (PBS), the main light-harvesting antennas for cyanobacteria and red algae, are located on the thylakoid membrane matrix side and are the largest light-harvesting protein complex to date. PBS transfers captured light energy to the reaction centers of photosystem II (PSII) and photosystem I (PSI) with extremely high efficiency through internal pigment groups (bilins) to induce the conversion of photo-chemical energy. While the structure of most photosynthetic protein complexes has been resolved by in vitro purification, the way in which they interact with each other and the pathways of energy transfer in their native state within cells are unknown.

The research group of Academician Sui Senfang of the School of Life Sciences of Tsinghua University has been committed to the research of phycobiliary body structure. In 2017, the team first reported in Nature the 3.5° resolution cryo-EM structure of the intact phycobilia of the marine red algae Griffithsia pacifica, revealing the precise assembly mechanism of the phycobilinosome. In 2020, the team reported in Nature the 2.8° resolution cryo-EM structure of intact phycobilia of Porphyridium purpureum in the journal Nature, revealing the mechanism of energy transfer in phycobilinosomes, which is a major research achievement in this field.

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Figure 1.Structure of the in situ PBS-PSII-PSI-LHC supercomplex

On March 15, Academician Sui Senfang and Professor Wang Hongwei, School of Life Sciences, Tsinghua University/Beijing Center for Advanced Innovation in Structural Biology/Beijing Center for Biological Structure Frontier Research, and researcher Zhang Xinzheng of the Institute of Biophysics of the Chinese Academy of Sciences, jointly published in the journal Nature entitled “In situstructure of the red algal superstructure” phycobilisome-psii-psi-lhc megacomplex) laid a solid structural foundation for elucidating the assembly mechanism of PBS-PSII-PSI-LHC supercomplex in the native state of the cell, as well as the mechanism of efficient energy transfer from PBS to PSII and PSI, which is a milestone achievement in the field of photosynthesis.

On the basis of previous phycobilisome research, the research team selected Shiozawa red algae as the research object for the analysis of in situ high-resolution structure, by combining cryofocused ion beam (cryo-FIB), cryo-ET, and subtomogram technology averaging) and the in situ single-particle analysis method (isSPA) resolved the in situ structures of two conformations (single-PBS-PSII-PSI-LHC and double-PBS-PSII-PSI-LHC) of the PBS-PSII-PSI-LHC with resolutions of 3.3 and 4.3?, respectively (Figure 1). Among these structures, the researchers found that protein molecules that had not been observed in samples isolated and purified into vitro played a crucial role in the assembly of PBS-PSII-PSI-LHC supercomplexes. Structural analysis showed that four connexins (LRC2, LRC3, LPP1 and LPP2) in PBS were involved in the binding between PBS-PSII together with LCM and APCD, so red algae PBS could bind to PSII very stably in vivo.

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Figure 2.Pigment arrangement in an in-situ PBS-PSII-PSI-LHC oversized complex

Photosynthetic organisms have evolved a response mechanism to changing light conditions in nature. When the excitation energy obtained by PSII is too high, in order to avoid PSII damage, the light-catching antenna will transfer energy to PSI, thereby redistributing energy. It is currently believed that the PBS of cyanobacteria and red algae may transmit energy to PSI through indirect PBS→PSII→PSI mode and direct PBS→PSI mode. However, due to the lack of structural information, the energy transfer mechanism between PBS and PSI has been controversial. The in-situ structure of the PBS-PSII-PSI-LHC complex now resolved shows that red algae PBS only interacts directly with PSII, so the captured light energy is first transferred to PSII and then to PSI through PSII, which is consistent with the energy spillover model (Figure 2). In addition, the researchers also found that there are specially arranged chlorophyll (chl) clusters in PSII and PSI, and their tight arrangement promotes the excited state coupling effect of π electrons on the conjugate ring, enabling the reduction of the energy level to ensure efficient energy transfer (Figure 2). In summary, the results prove that red algae transfer energy to PSI through indirect PBS→PSII→PSI mode, thereby redistributing energy for PSII and PSI. The results also provide a new theoretical basis for the study of artificial simulation of photosynthesis.

You Xin, a 2019 doctoral student at the School of Life Sciences of Tsinghua University, Xing Zhang, a postdoctoral fellow at the Institute of Biophysics of the Chinese Academy of Sciences, Cheng Jing, and a postdoctoral fellow at Southern University of Science and Technology, Xiao Yanan, are the co-first authors of the paper, and Academician Sui Senfang and Professor Wang Hongwei of the School of Life Sciences of Tsinghua University and Zhang Xinzheng, a researcher at the Institute of Biophysics of the Chinese Academy of Sciences, are the co-corresponding authors of the paper. Ma Jianfei, a postdoctoral fellow at Tsinghua University, conducted preliminary exploration, and associate researcher Sun Shan participated in the structural analysis.

The cryo-EM platform, computing platform and mass spectrometry platform of Tsinghua Base of National Protein Science Research (Beijing) Facility, the Analysis and Testing Center of Southern University of Science and Technology and the Public Technology Center of the Institute of Botany, Chinese Academy of Sciences provided important technical support for the study, and Shen Jianren, a researcher at the Institute of Botany, Chinese Academy of Sciences, provided useful discussions for the study. The research has been supported by the National Natural Science Foundation of China, the National Key Research and Development Program, the Beijing Frontier Research Center for Biological Structures, the Beijing Advanced Innovation Center for Structural Biology, and the Tencent Foundation.

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