Structural insights into an evolutionary turning-point of photosystem I from prokaryotes to eukaryotes

Abstract

Photosystem I (PSI) contributes to light-conversion reactions; however, its oligomerization state is variable among photosynthetic organisms. Herein we present a 3.8-Å resolution cryo-electron microscopic structure of tetrameric PSI isolated from a glaucophyte alga Cyanophora paradoxa. The PSI tetramer is organized in a dimer of dimers form with a C2 symmetry. Different from cyanobacterial PSI tetramer, two of the four monomers are rotated around 90°, resulting in a totally different pattern of monomer-monomer interactions. Excitation-energy transfer among chlorophylls differs significantly between Cyanophora and cyanobacterial PSI tetramers. These structural and spectroscopic features reveal characteristic interactions and energy transfer in the Cyanophora PSI tetramer, thus offering an attractive idea for the changes of PSI from prokaryotes to eukaryotes.

Algal photosystem I dimer and high resolution model of PSI:plastocyanin complex

“…As a short-term light acclimation mechanism in response to fluctuating illumination and anoxia, the algal PSI additionally associates with two LHCII trimers 5,6 . Structural studies have shown that the oligomeric state of a chloroplast PSI is a monomer, due to the presence of the subunit PsaH, whereas in cyanobacteria structures of dimers [7][8][9][10] and trimers 11 were also reported. Cyanobacterial PSI oligomerizes via direct contacts between subunits PsaI and PsaL, however such an association has been ruled out for a chloroplast PSI due to structural constraints of PsaH presence that imposes an apparent rigidity 12,13 .…”

“…2a). In cyanobacteria, PSI dimerises via the stromal region of PsaL 7–10 and trimerises via the lumenal C-terminus of PsaL, assisted by PsaI 11 . In our structure of the chloroplast PSI-LHCI dimer, neither PsaL nor PsaI interacts with each other between the neighbouring units.…”

Cryomicroscopy in situ: what is the smallest molecule that can be directly identified without labels in a cell?

“…CryoEM maps of all these 16 protein complexes (A) were released in the EMDB 20 within a single week (10-16 February 2022). [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] The total number of structures released in that week was 121. Here, only some of these structures, whose resolution reached better than 4 Å, and for which atomic models were deposited in the PDB, are shown.…”