In situ structure of the red algal phycobilisome-PSII-PSI-LHC megacomplex

In oxygenic photosynthetic organisms, light energy is captured by antenna systems and transferred to photosystem II (PSII) and photosystem I (PSI) to drive photosynthesis(1,2). The antenna systems of red algae consist of soluble phycobilisomes (PBSs) and transmembrane light-harvesting complexes (LHCs)(3). Excitation energy transfer pathways from PBS to photosystems remain unclear owing to the lack of structural information. Here we present in situ structures of PBS-PSII-PSI-LHC megacomplexes from the red alga Porphyridium purpureum at near-atomic resolution using cryogenic electron tomography and in situ single-particle analysis(4), providing interaction details between PBS, PSII and PSI. The structures reveal several unidentified and incomplete proteins and their roles in the assembly of the megacomplex, as well as a huge and sophisticated pigment network. This work provides a solid structural basis for unravelling the mechanisms of PBS-PSII-PSI-LHC megacomplex assembly, efficient energy transfer from PBS to the two photosystems, and regulation of energy distribution between PSII and PSI.

Automated summary

In this study, in situ structures of PBS-PSII-PSI-LHC megacomplexes from the red alga Porphyridium purpureum were determined at near-atomic resolution. The structures revealed the interaction details between PBS, PSII, and PSI, as well as a large and complex pigment network. This work provides a solid structural basis for understanding the assembly mechanism, efficient energy transfer, and energy distribution regulation of the PBS-PSII-PSI-LHC megacomplex.