Chloroplast photosystem I dimer and high resolution model of the complex with plastocyanin

Photosystem 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. Our cryo-EM structure identifies the light-harvesting protein Lhca9 as the key element for the dimerization. Furthermore, the absence of Lhca2 and PsaH, gives rise to a head-to-head relative orientation of the PSI-LHCI monomers, in a way that is essentially different from the oligomer formation in cyanobacteria. The interface between the monomers partially overlaps with the surface area that would bind one of the LHCII complexes in state transitions. We also define the most accurate available PSI-LHCI model at 2.3 Å resolution, including a flexibly bound electron donor plastocyanin, and assign correct identities and orientations of all the pigments, as well as 486 water molecules that affect energy transfer pathways. ### Competing Interest Statement The authors have declared no competing interest.