The structure of phycobilisome with a bicylindrical core from the cyanobacterium Synechococcus elongatus PCC 7942

Phycobilisomes (PBSs) are the major light-harvesting complexes in the cyanobacteria and red algae and they consist of a central core and peripheral rods that are attached to the core. The PBS cores contain 2-5 allophycocyanin cylinders that are organized by ApcE. At the present, structures of PBS with tricylindrical and pentacylindrical cores have been determined while the structure of the PBS with a bicylindrical core is yet to be revealed. Here we report the cryo-EM structure of PBS with bicylindrical core from Synechococcus elongatus PCC 7942 (Synechococcus 7942) at an overall resolution of approximately 3 Å. Similar to the PBS with a tricylindrical core, six peripheral rods are attached to the core by the rod-core linker protein CpcG in the PBS of Synechococcus 7942 even though the core lacks the top AP cylinder, which is important for the attachment of peripheral rods to the tricylindrical cores. We found that the C-terminus of ApcE in the Synechococcus 7942 was involved in interacting with both CpcG and CpcB of a top peripheral rod, compensating for the absence of the top AP cylinder of the core and maintaining PBS stability. Analysis of the bilin distribution reveals that distance of excitation energy transfer from top peripheral rods to the terminal emitters is approximately 15% shorter compared to the PBS with tricylindrical cores. Although there are 30% fewer bilin chromophores in the Synechococcus 7942 PBS core compared with the tricylindrical core, the aromatic residue ring in the Synechococcus 7942 PBS core is conserved, supporting the suggestion that these aromatic residues from AP and linker proteins are critical to the energy transfer of PBS.