The phycobilisome core-membrane linkers from Synechocystis sp. PCC 6803 and red-algae assemble in the same topology

The phycobilisomes (PBSs) of cyanobacteria and red-algae are unique megadaltons light-harvesting protein-pigment complexes that utilize bilin derivatives for light absorption and energy transfer. Recently, the high-resolution molecular structures of red-algal PBSs revealed how the multi-domain core-membrane linker (L-CM) specifically organizes the allophycocyanin subunits in the PBS's core. But, the topology of L-CM in these structures was different than that suggested for cyanobacterial PBSs based on lower-resolution structures. Particularly, the model for cyanobacteria assumed that the Arm2 domain of L-CM connects the two basal allophycocyanin cylinders, whereas the red-algal PBS structures revealed that Arm2 is partly buried in the core of one basal cylinder and connects it to the top cylinder. Here, we show by biochemical analysis of mutations in the apcE gene that encodes L-CM, that the cyanobacterial and red-algal L-CM topologies are actually the same. We found that removing the top cylinder linker domain in L-CM splits the PBS core longitudinally into two separate basal cylinders. Deleting either all or part of the helix-loop-helix domain at the N-terminal end of Arm2, disassembled the basal cylinders and resulted in degradation of the part containing the terminal emitter, ApcD. Deleting the following 30 amino-acids loop severely affected the assembly of the basal cylinders, but further deletion of the amino-acids at the C-terminal half of Arm2 had only minor effects on this assembly. Altogether, the biochemical data are consistent with the red-algal L-CM topology, suggesting that the PBS cores in cyanobacteria and red-algae assemble in the same way.