Structural Superimposition

Abstact

Photosynthesis sustains life on Earth by converting light to chemical energy through the coordinated action of photosystem I (PSI) and photosystem II (PSII) within thylakoid membranes(1-4). Although structures of isolated photosystems are available, their native organization in chloroplasts remains unknown. Here, using in situ cryo-electron microscopy, we directly imaged Oryza sativa (rice) chloroplasts and determined structures of photosystem supercomplexes in their native membrane environment. We resolved a C(2)S(2)M(2)L(4)-type PSII-light harvesting complex II (LHCII) supercomplex, including four LHCII antenna trimers that were not retained in purified preparations. Excitation energy transfer calculations based on this architecture closely reproduce in vivo measurements, indicating its physiological relevance. We also resolved asymmetric PSII-LHCII dimers, including side-by-side, trans-lumenal and trans-stromal architectures, and higher-order assemblies of trimers and tetramers. On the basis of these observations, we propose that PSII forms a trans-lumenal and trans-stromal 'skeleton' that shapes thylakoid morphology and supports grana stacking. In addition, we obtained high-resolution structures of PSI-LHCI-LHCII and PSI-LHCI supercomplexes. Together, these structures reveal extensive networks of lipids, pigments and cofactors, providing the first molecular framework for understanding how the native architecture of plant photosystem supports the exceptional photon-to-electron efficiency of photosynthesis.