The photosynthesis machinery in chloroplast thylakoid membranes is comprised of multiple protein complexes and supercomplexes1,2. Here, we show a novel supramolecular organization of photosystem I (PSI) in the moss Physcomitrella patens by single-particle cryo-electron microscopy. The moss-specific light-harvesting complex (LHC) protein Lhcb9 is involved in this PSI supercomplex, which has been shown to have a molecular density similar to that of the green alga Chlamydomonas reinhardtii3. Our results show that the structural organization is unexpectedly different—two rows of the LHCI belt exist as in C. reinhardtii4, but the outer one is shifted toward the PsaK side. Furthermore, one trimeric LHC protein and one monomeric LHC protein position alongside PsaL/K, filling the gap between these subunits and the outer LHCI belt. We provide evidence showing that Lhcb9 is a key factor, acting as a linkage between the PSI core and the outer LHCI belt to form the unique supramolecular organization of the PSI supercomplex in P. patens.
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The 3D cryo-EM density map of Pp-PSI-L has been deposited in the Electron Microscopy Data Bank under accession code EMD-9107, and with the Protein Data Bank (PDB) under accession code 6MEM. The data are available from the corresponding authors (M.I. and K.K.N.) upon request.
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The QB3/Chemistry Mass Spectrometry Facility at the University of California, Berkeley receives support from the National Institutes of Health (grant 1S10OD020062-01). This work was supported by the US Department of Energy, Office of Science, through the Photosynthetic Systems programme in the Office of Basic Energy Sciences. E.N. and K.K.N. are investigators of the Howard Hughes Medical Institute.