Abstract
Phycobilisomes (PBS) are the major light-harvesting machineries for photosynthesis in cyanobacteria and red algae and they have a hierarchical structure of a core and peripheral rods, with both consisting of phycobiliproteins and linker proteins. Here we report the cryo-EM structures of PBS from two cyanobacterial species, Anabaena 7120 and Synechococcus 7002. Both PBS are hemidiscoidal in shape and share a common triangular core structure. While the Anabaena PBS has two additional hexamers in the core linked by the 4th linker domain of ApcE (LCM). The PBS structures predict that, compared with the PBS from red algae, the cyanobacterial PBS could have more direct routes for energy transfer to ApcD. Structure-based systematic mutagenesis analysis of the chromophore environment of ApcD and ApcF subunits reveals that aromatic residues are critical to excitation energy transfer (EET). The structures also suggest that the linker protein could actively participate in the process of EET in both rods and the cores. These results provide insights into the organization of chromophores and the mechanisms of EET within cyanobacterial PBS.
Highlights
Phycobilisomes (PBS) are the major light-harvesting machineries for photosynthesis in cyanobacteria and red algae and they have a hierarchical structure of a core and peripheral rods, with both consisting of phycobiliproteins and linker proteins
The major light-harvesting system for solar energy capture in cyanobacteria and red algae is phycobilisome (PBS)[4,5,6,7,8], which consists of phycobiliproteins (PBP) with covalently attached open-chain tetrapyrroles as chromophores and linker proteins[5,9,10]
Negativestaining electron microscopy analysis revealed a large variation in the length of peripheral rods (Supplementary Fig. 2a, b), and the longest rods could contain as many as six PC hexamers
Summary
The structural comparison indicates that the equivalent layers of A1/A'1 in the basal cylinders and layers of B1/B1’ in the top cylinder of the cyanobacterial PBS are lost in the red algal PBS (Supplementary Fig. 2c–e), likely reflecting an adaptation to the environment of deep ocean water where the more blue–green light is available[11,12]. Despite these differences, the general spatial distributions of ApcE, ApcD and ApcF in the core are the same in the cyanobacteria and red algae PBS. The determination of the core structures of PBS from these cyanobacteria, especially the one from high light tolerant Synechococcus 7002, will help future studies of the mechanism of OCPs
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