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The plant photosystem I (PSI) supercomplex at 3.4 Å resolution (Amunts et al., 2007). It contains 4 light harvesting chlorophyll a/b binding proteins as well as 13 additional constituents. One helix (TMS) proteins, OHP1 (O81208) amd OHP2 (Q9FEC1) play an essential role in the assembly or stabilization of photosynthetic pigment-protein complexes, especially photosystem reaction centers, in the thylakoid membrane (Beck et al. 2017). PSI consists of two complexes, a reaction center and light-harvesting complex (LHC), which together form the PSI-LHC supercomplex. The crystal structure of plant PSI has been solved with two distinct crystal forms. The first, crystallized at pH 6.5, exhibited P21 symmetry; the second, crystallized at pH 8.5, exhibited P212121 symmetry. The surfaces involved in binding plastocyanin and ferredoxin were identical in both forms. The crystal structure at 2.6 Å resolution revealed 16 subunits, 45 transmembrane helices, and 232 prosthetic groups, including 143 chlorophyll a, 13 chlorophyll b, 27 beta-carotene, 7 lutein, 2 xanthophyll, 1 zeaxanthin, 20 monogalactosyl diglyceride, 7 phosphatidyl diglyceride, 5 digalactosyl diglyceride, 2 calcium ions, 2 phylloquinone, and 3 iron sulfur clusters (Caspy and Nelson 2018). The model revealed detailed interactions, providing mechanisms for excitation energy transfer and its modulation in one of nature's most efficient photochemical machine. The photoexcitation response of cyanobacterial Photosystem I has been studied following reconstitution in proteoliposomes (Niroomand et al. 2017).

Photosystem I of Arabidopsis thaliana
(PsaA-L; Lhca 1-4)
PsaA (chlorophyll a apoprotein; 750 aas; 7 TMSs) (P56766)
PsaB (chlorophyll a apoprotein; 734 aas; 11-12 TMSs) (P56767)
PsaC (iron sulfur center protein) (P62090)
PsaD (Reaction Center Subunit II) (Q9SA56)
PsaE (Reaction Center Subunit IV) (Q9S714)
PsaF (Reaction Center Subunit III) (Q9SUI8)
PsaG (Reaction Center Subunit V) (Q9S7N7)
PsaH (Reaction Center Subunit VI) (Q9SUI6)
PsaI (Reaction Center Subunit VIII) (P56768)
PsaJ (Reaction Center Subunit IX) (P56769)
PsaK (Reaction Center Subunit X) (Q9SUI5)
PsaL (Reaction Center Subunit XI) (Q9SUI4)
Lhca1 (225 aas; 1-2 TMSs) (ABD37878)
Lhca2 (257 aas; 1-2 TMSs) (Q9SYW8)
Lhca3 (273 aas; 1-2 TMSs) (Q43381)
Lhca4 (244 aas; 2-4 TMSs) (Q6YWJ7)