In situ structural determination of cyanobacterial phycobilisome-PSII supercomplex by STAgSPA strategy

Photosynthesis converting solar energy to chemical energy is one of the most important chemical reactions on earth. In cyanobacteria, light energy is captured by antenna system phycobilisomes (PBSs) and transferred to photosynthetic reaction centers of photosystem II (PSII) and photosystem I (PSI). While most of the protein complexes involved in photosynthesis have been characterized by in vitro structural analyses, how these protein complexes function together in vivo is not well understood. Here we developed an in situ structural analysis strategy ″STAgSPA″ to successfully solve the in situ structure of PBS-PSII supercomplex from the cyanobacteria Spirulina platensis FACHB-439 at resolution of 3.5?. The structure reveals the unprecedented coupling details among adjacent PBSs and PSII dimers, and the collaborative energy transfer mechanism mediated by multiple ″super-PBS″ in cyanobacteria. Our results not only provide the insights for understanding the diversity of photosynthesis-related systems between prokaryotic cyanobacteria and eukaryotic red algae, but also a valuable methodological demonstration for in situ high-resolution structural analysis in cellular or tissue samples.