Assembly and Disassembly of Phycobilisomes

The process of photosynthesis is initiated by the absorption of light energy by large arrays of pigmentsbound in an ordered fashion within protein complexes called antennas. These antennas transfer the absorbedenergy at almost 100% efficiency to the reaction centers that perform the photochemical electron transferreactions required for the conversion of the light energy into useful and storable chemical energy. Inprokaryotic cyanobacteria, eukaryotic red algae and cyanelles, the major antenna complex is called the phycobilisome,an extremely large (3–7 MDa) multi subunit complex found on the stromal side of the thylakoidmembrane. Phycobilisomes are assembled in an ordered sequence from similarly structured units that covalentlybind a variety of linear tetrapyrolle pigments called bilins. Phycobilisomes have a broad cross-sectionof absorption (500–680 nm) and mainly transfer the absorbed energy to photosystem II. Theycan, however, function as an antenna of photosystem I, and their composition can be altered as a resultof changes in the environmental light quality. The phycobilisome is structurally and functionally differentfrom other classes of photosynthetic antenna complexes. In this review, we will describe the importantstructural and functional characteristics of the phycobilisome complex and its components, especially withrespect to its assembly and disassembly.