Unique organization of photosystem I–light-harvesting supercomplex revealed by cryo-EM from a red alga

Xiong Pi, Lirong Tian, Huai En Dai, Xiaochun Qin, Lingpeng Cheng, Tingyun Kuang, Sen Fang Sui, Jian-Ren Shen

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31 Citations (Scopus)

Abstract

Photosystem I (PSI) is one of the two photosystems present in oxygenic photosynthetic organisms and functions to harvest and convert light energy into chemical energy in photosynthesis. In eukaryotic algae and higher plants, PSI consists of a core surrounded by variable species and numbers of light-harvesting complex (LHC)I proteins, forming a PSI-LHCI supercomplex. Here, we report cryo-EM structures of PSI-LHCR from the red alga Cyanidioschyzon merolae in two forms, one with three Lhcr subunits attached to the side, similar to that of higher plants, and the other with two additional Lhcr subunits attached to the opposite side, indicating an ancient form of PSI-LHCI. Furthermore, the red algal PSI core showed features of both cyanobacterial and higher plant PSI, suggesting an intermediate type during evolution from prokaryotes to eukaryotes. The structure of PsaO, existing in eukaryotic organisms, was identified in the PSI core and binds three chlorophylls a and may be important in harvesting energy and in mediating energy transfer from LHCII to the PSI core under state-2 conditions. Individual attaching sites of LHCRs with the core subunits were identified, and each Lhcr was found to contain 11 to 13 chlorophylls a and 5 zeaxanthins, which are apparently different from those of LHCs in plant PSI-LHCI. Together, our results reveal unique energy transfer pathways different from those of higher plant PSI-LHCI, its adaptation to the changing environment, and the possible changes of PSI-LHCI during evolution from prokaryotes to eukaryotes.

Original languageEnglish
Pages (from-to)4423-4428
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume115
Issue number17
DOIs
Publication statusPublished - Apr 24 2018

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Keywords

  • Cryo-EM
  • Energy transfer
  • PsaO
  • PSI-LHCR
  • Red algae

ASJC Scopus subject areas

  • General

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