Abstract
Photosystem II (PSII) generates an oxidant whose redox potential is high enough to enable water oxidation (Shen 2015; McEvoy and Brudvig 2006), a substrate so abundant that it assures a practically unlimited electron source for life on earth (Barber 2004). Our knowledge on the mechanism of water photooxidation was greatly advanced by high-resolution structures of prokaryotic PSII (Umena et al. 2011; Suga et al. 2019; Kato et al. 2021). Here we show high-resolution cryo-EM structures of eukaryotic PSII from the green alga Dunaliella salina at two distinct conformations. The conformers are also present in stacked PSII, exhibiting flexibility that may relevant to the grana formation in chloroplasts of the green lineage. CP29, one of PSII associated light-harvesting antennae, plays a major role in distinguishing the two conformations of the supercomplex. We also show that the stacked PSII dimer, a form suggested to support the organization of thylakoid membranes (Garab and Mustárdy 2000; Kirchhoff et al. 2007), can appear in many different orientations providing a flexible stacking mechanism for the arrangement of grana stacks in thylakoids. Our findings provide a structural basis for the heterogenous nature of the eukaryotic PSII on multiple levels.
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