This review focuses on the Orange Carotenoid Protein (OCP) which is the first photoactive protein identified containing a carotenoid as the photoresponsive chromophore. This protein is essential for the triggering of a photoprotective mechanism in cyanobacteria which decreases the excess absorbed energy arriving at the photosynthetic reaction centers by increasing thermal dissipation at the level of the phycobilisomes, the cyanobacterial antenna. Blue-green light causes structural changes within the carotenoid and the protein, converting the orange inactive form into a red active form. The activated red form interacts with the phycobilisome and induces the decrease of phycobilisome fluorescence emission and of the energy arriving to the photosynthetic reaction centers. The OCP is the light sensor, the signal propagator and the energy quencher. A second protein, the Fluorescence Recovery Protein (FRP), is needed to detach the red OCP from the phycobilisome and its reversion to the inactive orange form. In the last decade, in vivo and in vitro mechanistic studies combined with structural and genomic data resulted in both the discovery and a detailed picture of the function of the OCP and OCP-mediated photoprotection. Recent structural and functional results are emphasized and important previous results will be reviewed. Similarities to other blue-light responsive proteins will be discussed.
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