Preparation, structural characterization, and ultrafast energy transfer dynamics of the phycobilisome-photosystem II megacomplex in a thermophilic cyanobacterium.

Phycobilisome (PBS) is a water-soluble light-harvesting supercomplex found in cyanobacteria, glaucophytes, and rhodophytes. PBS interacts with photosynthetic reaction centers, specifically photosystems II and I (PSII and PSI), embedded in the thylakoid membrane. It is widely accepted that PBS predominantly associates with PSII, which functions as the initial complex in the linear electron transport chain. Structures of various types of PBSs with different morphologies and/or absorption properties have been reported using cryo-electron microscopy and X-ray crystallography. However, the detailed energy transfer process between PBS and PSII remains to be elucidated due to the lack of a reliable preparation method for PBS-PSII megacomplexes, in which PBS and PSII interact with each other. In this study, we established a new method for isolating the PBS-PSII megacomplex using ammonium sulfate and dodecyl-α-D-maltoside as a stabilizing reagent and a detergent, respectively. In addition, we evaluated the detailed energy transfer mechanism in the PBS-PSII megacomplex, revealing the rate constants of the funnel-type excitation energy transfer from PBS to PSII. The method will enhance our understanding of the biochemical properties and energy transfer dynamics of diverse PBS-PSII megacomplexes.