Structures of the Cyanobacterial Phycobilisome
The phycobilisome is an elaborate antenna that is responsible for light-harvesting in cyanobacteria and red-algae. This large macromolecular complex captures incident sunlight and transfers the energy via a network of pigment molecules called bilins to the photosynthetic reaction centers. The phycobilisome of the model organism Synechocystis PCC 6803 consists of a core to which six rods are attached but its detailed molecular architecture and regulation in response to environmental conditions is not well understood. Here we present cryo-electron microscopy structures of the 6.2 MDa phycobilisome from Synechocystis PCC 6803 resolved at 2.1 [A] (rods) to 2.7 [A] (core), revealing three distinct conformations, two previously unknown. We found that two of the rods are mobile and can switch conformation within the complex, revealing a layer of regulation not described previously. In addition, we found a novel linker protein in the structure, that may represent a long-sought subunit that tethers the phycobilisome to the thylakoid membrane. Finally, we show how excitation energy is transferred within the phycobilisome and correlate our structures with known spectroscopic properties. Together, our results provide detailed insights into the biophysical underpinnings of cyanobacterial light harvesting and lay the foundation for bioengineering of future phycobilisome variants and artificial light harvesting systems.