Isolation of intact phycobilisomes in low salt: A novel method for purifying phycobilisomes by mild cross-linking

Photosynthesis is initiated by the capture of light energy, absorbed and transferred to photosynthetic reaction centers by light harvesting complexes. In cyanobacteria and red algae the major light harvesting complex is the Phycobilisome. The phycobilisome is a gigantic photosynthetic antenna with a molecular weight between 3–7 MDa and dimensions of 40–60 nm. The complex is assembled by two multi-subunit sub-complexes: A central core which is surrounded by rods in which the energy is transferred from the distal side of the rod to the core. In order to isolate the entire complex, the presence of high concentration of phosphate buffer (> 0.75 mol) is necessary, to prevent immediate complex disassemble into subunits. We have developed a mild procedure of cross-linking the phycobilisome from the thermophilic cyanobacterium Thermosynechococcus vulcanus that enables us to obtain an intact and functional complex under low ionic strength conditions. The cross-linked complex was examined by a spectroscopic analysis that confirmed that the isolated complex was indeed intact and continues to transfer energy from the rod to the core.