Structure of Dunaliella Photosystem II reveals conformational flexibility of stacked and unstacked supercomplexes

Photosystem II (PSII) generates an oxidant whose redox potential is high enough to enable water oxidation^1,2, a substrate so abundant that it assures a practically unlimited electron source for life on earth³. Our knowledge on the mechanism of water photooxidation was greatly advanced by high-resolution structures of prokaryotic PSII^4–6. Here we show high-resolution structures of eukaryotic PSII from the green algae Dunaliella salina at two distinct conformations. The conformers are also present in stacked PSII, exhibiting flexibility that is 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^7,8, 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.