Photosynthetic reaction centers are sensitive to high light conditions, which can cause damage because of the formation of reactive oxygen species. To prevent high-light induced damage, cyanobacteria have developed photoprotective mechanisms. One involves a photoactive carotenoid protein that decreases the transfer of excess energy to the reaction centers. This protein, the orange carotenoid protein (OCP), is present in most cyanobacterial strains; it is activated by high light conditions and able to dissipate excess energy at the site of the light-harvesting antennae, the phycobilisomes. Restoration of normal antenna capacity involves the fluorescence recovery protein (FRP). The FRP acts to dissociate the OCP from the phycobilisomes by accelerating the conversion of the active red OCP to the inactive orange form. We have determined the 3D crystal structure of the FRP at 2.5 Å resolution. Remarkably, the FRP is found in two very different conformational and oligomeric states in the same crystal. Based on amino acid conservation analysis, activity assays of FRP mutants, FRP:OCP docking simulations, and coimmunoprecipitation experiments, we conclude that the dimer is the active form. The second form, a tetramer, may be an inactive form of FRP. In addition, we have identified a surface patch of highly conserved residues and shown that those residues are essential to FRP activity. nonphotochemical quenching | Synechocystis L ight is vital for the survival and growth of photosynthetic organisms. In natural environments, these organisms are exposed to varying light conditions in addition to the day/night cycle. Too much exposure to light causes the formation of reactive oxygen species that damage the sensitive photochemical reaction centers, and thus, a careful regulation of energy flow is critical. Under low light conditions, an efficient energy collection by the antennae complexes is achieved, whereas under high light conditions, the excess energy has to be diverted from photosynthesis (1).Plants and cyanobacteria have evolved different ways to deal with excess energy arriving at the reaction centers. Higher plants and green algae contain antenna complexes consisting of transmembrane proteins that sense the acidification of the thylakoid lumen and react by switching from efficient energy collection to heat dissipation (1, 2). Cyanobacteria, however, contain antenna complexes called phycobilisomes, which are membrane-anchored and consist of phycobilin proteins. Instead of sensing the effect of high light through pH changes, cyanobacterial phycobilisomes are quenched by a protein capable of directly sensing high light conditions, the orange carotenoid protein (OCP). The 35 kDa OCP consists of two distinct domains that encompass a ketocarotenoid (3′-hydroxyechinenone) in an all trans conformation (3-6). Irradiance with high light changes the OCP from an inactive orange (OCP o ) to an active red form (OCP r ) that is capable of binding to the phycobilisomes to prevent excess energy from flowing to the reaction centers. ...

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