The structure of intact and active Photosystem <scp>II</scp> from <i>Arabidopsis thaliana</i> at 2.44 Å resolution

Photosystem II (PS II) is a large membrane-bound protein complex that catalyses light-driven water oxidation in plants and cyanobacteria. The structure of PS II is well studied in cyanobacteria; however, there are very few PS II structures from plants. The currently available plant PS II structures are comparatively low resolution and are frequently incomplete, that is, missing subunits or cofactors. We optimized the procedure for isolating PS II from Arabidopsis thaliana and employed cryo-electron microscopy to generate a high-resolution structure of an intact and oxygen-evolving PS II from Arabidopsis thaliana at 2.44 Å resolution, which to date represents the highest resolution structure of PS II from higher plants. At this resolution, many water molecules within the PS II structure can be detected, including waters around the water-splitting manganese cluster, the nonheme iron, and within the water/proton channels connecting these active sites to the protein exterior, allowing for the first detailed description of the water networks in Arabidopsis thaliana and comparison with the highly resolved cyanobacterial PS II. Our findings further the understanding of design principles of protein-water-cofactor interactions in photosynthetic water splitting, quinone reduction/exchange, and about the role of lipids at the interface between PS II and the light-harvesting proteins.