Cyanobacterial photoprotection by the orange carotenoid protein.

In photosynthetic organisms, the production of dangerous oxygen species is stimulated under high irradiance. To cope with this stress, these organisms have evolved photoprotective mechanisms. One type of mechanism functions to decrease the energy arriving at the photochemical centres by increasing thermal dissipation at the level of antennae. In cyanobacteria, the trigger for this mechanism is the photoactivation of a soluble carotenoid protein, the orange carotenoid protein (OCP), which is a structurally and functionally modular protein. The inactive orange form (OCPo) is compact and globular, with the carotenoid spanning the effector and the regulatory domains. In the active red form (OCPr), the two domains are completely separated and the carotenoid has translocated entirely into the effector domain. The activated OCPr interacts with the phycobilisome (PBS), the cyanobacterial antenna, and induces excitation-energy quenching. A second protein, the fluorescence recovery protein (FRP), dislodges the active OCPr from the PBSs and accelerates its conversion to the inactive OCP.