Structure of phycobilisome from the red alga Griffithsia pacifica

Life on Earth depends on photosynthesis for its conversion of solar energy to chemical energy. Photosynthetic organisms have developed a variety of light-harvesting systems to capture sunlight. The largest light-harvesting complex is the phycobilisome (PBS), the main light-harvesting antenna in cyanobacteria and red algae. It is composed of phycobiliproteins and linker proteins but the assembly mechanisms and energy transfer pathways of the PBS are not well understood. Here we report the structure of a 16.8-megadalton PBS from a red alga at 3.5 Å resolution obtained by single-particle cryo-electron microscopy. We modelled 862 protein subunits, including 4 linkers in the core, 16 rod–core linkers and 52 rod linkers, and located a total of 2,048 chromophores. This structure reveals the mechanisms underlying specific interactions between linkers and phycobiliproteins, and the formation of linker skeletons. These results provide a firm structural basis for our understanding of complex assembly and the mechanisms of energy transfer within the PBS. Single-particle cryo-electron microscopy is used to resolve the structure of the phycobilisome, a 16.8-megadalton light-harvesting megacomplex, from the red alga Griffithsia pacifica at a resolution of 3.5 Å. Illuminating the phycobilisome The largest light-harvesting complex is a 16.8-megadalton megacomplex called the phycobilisome. Sen-Fang Sui and colleagues have used single-particle cryo-electron microscopy to solve the structure of this hemispherical complex from the red alga Griffithsia pacifica , visualizing 860 protein components and 2,048 chromophores—the parts of a molecule that cause it to be coloured. This structural achievement provides a mechanistic understanding of how the complex can accommodate changing light conditions and how energy transfer occurs.