Structure of a cyanobacterial photosystem I tetramer revealed by cryo-electron microscopy

Seiji Akimoto,Ryo Nagao,Naoyuki Miyazaki,Jian-Ren Shen,Masahiko Ikeuchi,Tian-Yi Jiang,Makio Yokono,Koji Kato,Mai Watanabe,Fusamichi Akita,Siu Kit Chan,Yoshifumi Ueno

doi:10.1038/s41467-019-12942-8

Abstract

Photosystem I (PSI) functions to harvest light energy for conversion into chemical energy. The organisation of PSI is variable depending on the species of organism. Here we report the structure of a tetrameric PSI core isolated from a cyanobacterium, Anabaena sp. PCC 7120, analysed by single-particle cryo-electron microscopy (cryo-EM) at 3.3 Å resolution. The PSI tetramer has a C2 symmetry and is organised in a dimer of dimers form. The structure reveals interactions at the dimer-dimer interface and the existence of characteristic pigment orientations and inter-pigment distances within the dimer units that are important for unique excitation energy transfer. In particular, characteristic residues of PsaL are identified to be responsible for the formation of the tetramer. Time-resolved fluorescence analyses showed that the PSI tetramer has an enhanced excitation-energy quenching. These structural and spectroscopic findings provide insights into the physiological significance of the PSI tetramer and evolutionary changes of the PSI organisations.

Highlights

Photosystem I (PSI) functions to harvest light energy for conversion into chemical energy
The structures of cyanobacterial PSI core trimer and monomer have been solved by X-ray crystallography[11,12,17], and the structures of monomeric PSI core from eukaryotic algae and higher plants have been solved by both X-ray crystallography and cryo-electron microscopy in the form of PSI-light harvesting complex I (LHCI) supercomplex[4,5,6,7,8,9,10]
The PSI core tetramers were prepared from Anabaena, and its biochemical and spectroscopic characterisations are summarised in Supplementary Fig. 1 together with PSI monomers and dimers obtained from the same cyanobacterium

Summary

Introduction

Photosystem I (PSI) functions to harvest light energy for conversion into chemical energy. The structures of cyanobacterial PSI core trimer and monomer have been solved by X-ray crystallography[11,12,17], and the structures of monomeric PSI core from eukaryotic algae and higher plants have been solved by both X-ray crystallography and cryo-electron microscopy (cryo-EM) in the form of PSI-light harvesting complex I (LHCI) supercomplex[4,5,6,7,8,9,10] These studies provide much information on the organisation of protein subunits, pigment arrangement and possible energy transfer pathways in the PSI core, as well as interactions among the PSI monomers in the cyanobacterial PSI trimer. The combination of structural and spectroscopic studies provides important clues on the structural and functional differences between the trimeric and tetrameric PSI cores as well as the possible evolutionary adaptations of tetrameric PSI to specific light environment

Methods

Results

Conclusion