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 remains unclear. The PBS has several peripheral rods and a central core that binds to the thylakoid membrane, allowing energy coupling with photosystem II (PSII) and PSI. Here, we have combined chemical cross-linking mass spectrometry with homology modeling to propose a tricylindrical 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 composed 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 and PSII provides increased accessibility for regulatory elements, e.g., orange carotenoid protein, which are required for modulating photochemical activity.

Automated summary

In cyanobacteria and red algae, the structural basis of efficient excitation energy transfer from the phycobilisome (PBS) antenna complex to the reaction centers is still unclear. A tricylindrical cyanobacterial PBS core structure was proposed using chemical cross-linking mass spectrometry and homology modeling, revealing the essential roles of anchoring domains in facilitating energy transfer to photosystem II (PSII) and PSI. The uneven bottom surface of the PBS core allows for increased accessibility for regulatory elements, such as the orange carotenoid protein, necessary for modulating photochemical activity.