Structure of Cyanobacterial Phycobilisome Core Revealed by Structural Modeling and Chemical Cross-linking

In cyanobacteria and red algae, the structural basis dictating efficient excitation energy transfer from the phycobilisome (PBS) antenna complex to the reaction centers (RCs) remains unclear. PBS has several peripheral rods and a central core, which binds to the thylakoid membrane, allowing energy coupling with Photosystems II (PSII) and Photosystem I (PSI). Here, we integrated chemical cross-linking mass spectrometry with homology modeling analysis to propose a tri-cylindrical cyanobacterial PBS-core structure. Our model reveals a side view crossover configuration of the two basal cylinders, consolidating the essential roles of the anchoring domains comprised of the ApcE PB-loop and ApcD, which facilitate the energy transfer to PSII and PSI respectively. The uneven bottom surface of the PBS-core contrasts with the flat reducing side of PSII. The extra space between two basal cylinders of the PBS-core and PSII provides increased accessibility of regulatory elements, e.g., orange carotenoid protein, which are required for modulating photochemical activities.