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Dimeric PSI of Chlamydomonas reinhardtii at 2.97 A resolution
Structural highlights
FunctionPSAA_CHLRE PsaA and PsaB bind P700, the primary electron donor of photosystem I (PSI), as well as the electron acceptors A0, A1 and FX. PSI is a plastocyanin/cytochrome c6-ferredoxin oxidoreductase, converting photonic excitation into a charge separation, which transfers an electron from the donor P700 chlorophyll pair to the spectroscopically characterized acceptors A0, A1, FX, FA and FB in turn. Oxidized P700 is reduced on the lumenal side of the thylakoid membrane by plastocyanin or cytochrome c6. Both potential cofactor branches in PSI seem to be active; however, electron transfer seems to proceed preferentially down the path including the phylloquinone bound by PsaA. Publication Abstract from PubMedPhotosystem I (PSI) enables photo-electron transfer and regulates photosynthesis in the bioenergetic membranes of cyanobacteria and chloroplasts. Being a multi-subunit complex, its macromolecular organization affects the dynamics of photosynthetic membranes. Here we reveal a chloroplast PSI from the green alga Chlamydomonas reinhardtii that is organized as a homodimer, comprising 40 protein subunits with 118 transmembrane helices that provide scaffold for 568 pigments. Cryogenic electron microscopy identified that the absence of PsaH and Lhca2 gives rise to a head-to-head relative orientation of the PSI-light-harvesting complex I monomers in a way that is essentially different from the oligomer formation in cyanobacteria. The light-harvesting protein Lhca9 is the key element for mediating this dimerization. The interface between the monomers is lacking PsaH and thus partially overlaps with the surface area that would bind one of the light-harvesting complex II complexes in state transitions. We also define the most accurate available PSI-light-harvesting complex I model at 2.3 A resolution, including a flexibly bound electron donor plastocyanin, and assign correct identities and orientations to all the pigments, as well as 621 water molecules that affect energy transfer pathways. Algal photosystem I dimer and high-resolution model of PSI-plastocyanin complex.,Naschberger A, Mosebach L, Tobiasson V, Kuhlgert S, Scholz M, Perez-Boerema A, Ho TTH, Vidal-Meireles A, Takahashi Y, Hippler M, Amunts A Nat Plants. 2022 Oct;8(10):1191-1201. doi: 10.1038/s41477-022-01253-4. Epub 2022 , Oct 13. PMID:36229605[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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