Solar energy harnessed by oxygenic photosynthesis is utilized to produce carbohydrates and molecular oxygen, providing the living foundations for almost all organisms on the earth. Oxygenic photosynthetic organisms (including plants, eukaryotic algae and cyanobacteria) contain two photosystems, namely photosystem I (PSI) and photosystem II (PSII). Light energy absorbed by the peripheral antennae of PSI and is used to drive electron transport to produce NADPH eventually. PSI is a multisubunit protein-pigment supercomplex containing two main moieties: the core complex and the peripheral antenna system. Core complexes have been well conserved during evolution, whereas the peripheral antenna system varies considerably from cyanobacteria to higher plants, and the green algal PSI is more versatile. Studies on C. reinhardtii (a model organism of green algae) showed that the antenna system from C. reinhardtii is larger and more complex than those from plants and red algae. C. reinhardtii PSI may bind up to ten light harvesting complex I (LHCI) proteins. In comparison, plant PSI core only stably binds four LHCIs, and a red algal PSI core associates with three or five LHCI proteins. Although the peripheral antenna system is larger and farther away from the core complex, time-resolved fluorescence analysis has indicated that C. reinhardtii PSI-LHCI has the same average lifetime as plant PSI-LHCI, indicating that the PS-LHCI of C. reinhardtii has a higher excitation energy transfer efficiency. So far, structures of PSI from red alga and higher plants were all determined, but the high resolution structure of green algal PSI remains unknown. Structural analysis on C. reinhardtii PSI-LHCI will facilitate deeper understanding of the high efficiency of light harvesting and energy transfer in green algal PSI-LHCI.

CHANG Wenrui - LI Mei’s group and ZHANG Xinzheng’s group from the Institute of Biophysics (IBP) at CAS collaborate and solved two cryo-electron microscopy (cryo-EM) structures of  C. reinhardtii PSI-LHCI supercomplex, with either eight or ten LHCIs associated with the PSI core(PSI-8LHCI or PSI-10LHCI), at an overall resolution of 2.9 angstrom (PSI-8LHCI) and 3.3 angstrom (PSI-10LHCI), respectively. In both structures, eight LHCIs are organized in two layers at one side of the PSI core. Each layer contains four LHCI proteins arranged in a crescent shape, forming the inner and outer LHCI belt. The subunit composition and the accurate locations of the eight antenna proteins are directly identified through means of structural biology for the first time. In the PSI-8LHCI structure, 18 protein subunits, 216 chlorophylls, 48 carotenoids and other cofactors were assigned. In the PSI-10LHCI structure, two additional LHCIs were found to attach to the opposite side of PSI core, and in total 23 protein subunits were identified. This is the largest PSI-LHCI structure (a 750 kDa membrane protein supercomplex) solved so far. The structures revealed that C. reinhardtii LHCIs in average bind more chlorophyll molecules compared with their counterparts in plants. Highly populated and closely associated chlorophylls in the antennae explain the high efficiency of light harvesting and excitation energy transfer in CrPSI–LHCI.

The research work, entitled "Antenna arrangement and energy transfer pathways of a green algal photosystem I-LHCI supercomplex" was published in Nature Plants on march 8, 2019. This research was supported by the National Key R&D Program of China, the Strategic Priority Research Program of CAS, the Key Research Program of Frontier Sciences of CAS, National Natural Science Foundation of China and the Youth Innovation Promotion Association of the CAS. The cryo-EM data was collected in Center for Biological Imaging, Institute of Biophysics, Chinese Academy of Sciences. The sample analysis work was assisted by the staffs in Chinese Academy of Sciences Protein Science Core Facility Center.

Figure. Structure of PS-LHCI supercomplex from C. reinhardtii.
A. PSI-8LHCI structure, B.PSI-10LHCI structure

(Imaged by Dr: Chang Wenrui - Li Mei’s group)

Article link: https://www.nature.com/articles/s41477-019-0380-5

Contact: Li Mei, Ph.D.
Principal Investigator

Institute of biophysics, Chinese Academy of Sciences
Beijing100101, China
Tel: (86)-10-64888511

Email: meili@ibp.ac.cn

(Reported by Dr. Chang Wenrui - Li Mei’s group)