Algae have indispensible roles in biosphere as they generate about 50% of the primary organic matters and account for ~50% of all oxygen production on the earth, through a fundamental biological process known as oxygenic photosynthesis. As a unicellular green algae, Chlamydomonas reinhardtii has been serving as an important model organism for photosynthesis research in the past decades. Moreover, it is also exploited as a biotechnological platform for production of high-value products such as biofuel and pharmaceutical compounds.

For algae and plants, oxygenic photosynthesis relies on photosystem II (PSII) and other supramolecular machineries to convert light energy into chemical energy. During the process, PSII catalyzes splitting of water molecules into oxygen and protons. To absorb photon energy efficiently, PSII assembles with the major light-harvesting complex II (LHCII) at the peripheral region. In C. reinhardtii, PSII recruits multiple LHCII complexes and they form various types of PSII-LHCII supercomplexes, including two major ones named C2S2 and C2S2M2L2 suercomplexes. Among them, the C2S2M2L2 supercomplex is the largest PSII-LHCII supercomplex ever found in algae and plants.

In a recent study led by Dr. LIU Zhenfeng from the Institute of Biophysics (IBP), Chinese Academy of Sciences and Dr. Jun Minagawa from the National Institute for Basic Biology in Japan, researchers solved the structures of Chlamydomonas C2S2 and C2S2M2L2-type PSII-LHCII supercomplexes through cryo-electron microscopy (cryo-EM) method at 2.7 and 3.4 angstrom resolution, respectively. The assembly mechanism and energy transfer pathways within the supercomplexes were deciphered in great details.

It was found for the first time that the LHCII trimer strongly associated (S-LHCII) with PSII core (C) contains three distinct subunits, namely LhcbM1, LhcbM2 and LhcbM3. Two special lipid molecules named digalactosyl diacylglycerol (or DGDG) mediate the assembly between LhcbM1 and PSII core antenna CP43. Furthermore, one pair of moderately-associated LHCII trimer (M-LHCII) and an additional pair of loosely-associated LHCII trimer (L-LHCII) attach at the peripheral region of C2S2 supercomplex, leading to the formation of the so-called C2S2M2L2 supercomplex.

By analyzing of the C2S2M2L2 supercomplex structure carefully, researchers found that the minor antenna complexes CP29 and CP26 contain several green-algae specific regions and these regions are absent in the homologs from land plants. The characteristic regions of Chlamydomonas CP29 have crucial roles in bridging L-LHCII with M-LHCII and stabilizing their assembly, whereas those of CP26 strengthen its own interactions with S-LHCII. Through quantitative analysis of the chlorophyll-chlorophyll relationships, multiple energy transfer routes from L-LHCII to PSII were unraveled and account for the fundamental steps of light-harvesting process in green algae. The results will improve our understanding on the molecular basis for efficient light harvesting and photoprotection in green algae under variable light conditions.

The work has been published online as an article entitled “Structural insights into light harvesting for photosystem II in green algae” in Nature Plants on Nov. 25th, 2019. The project was funded by the National Key R&D Program of China, National Natural Science Foundation of China, the Strategic Priority Research Program and the Key Research Program of Frontier Sciences of CAS, etc. The Cryo-EM data were collected at the Center for Biological Imaging at the Institute of Biophysics and on the Talos Arctica microscope sponsored by the National Key Laboratory of Biomacromolecules at IBP. SHENG Xin and LI Anjie from LIU Zhenfeng's group, Akimasa Watanabe and Eunchul Kim from Jun Minagawa Group, etc participate in this work. Dr. LIU Zhenfeng and Dr. Jun Minagawa are co-corresponding authors of the article. To read the full article, please click the following link at https://www.nature.com/articles/s41477-019-0543-4

Structures of C2S2 and C2S2M2L2-type PSII-LHCII supercomplex from a green algae (Chlamydomonas reinhardtii)
（Image by Dr. LIU Zhenfeng's group)

Article link: https://www.nature.com/articles/s41477-019-0543-4

Contact: LIU Zhenfeng
Institute of Biophysics, Chinese Academy of Sciences
Beijing 100101, China
Phone: 86-10-64881481
Email: liuzf@ibp.ac.cn

(Reported by Dr. LIU Zhenfeng's group)