Stages of OCP-FRP Interactions in the Regulation of Photoprotection in Cyanobacteria, Part 1: Time-Resolved Spectroscopy.

Most cyanobacteria utilize a water-soluble Orange Carotenoid Protein (OCP) to protect their light-harvesting complexes from photodamage. The Fluorescence Recovery Protein (FRP) is used to restore photosynthetic activity by inactivating OCP via dynamic OCP-FRP interactions, a multistage process that remains underexplored. In this work, applying time-resolved spectroscopy, we demonstrate that the interaction of FRP with the photoactivated OCP begins early in the photocycle. Interacting with the compact OCP state, FRP completely prevents the possibility of OCP domain separation and formation of the signaling state capable of interacting with the antenna. The structural element that prevents FRP binding and formation of the complex is the short α-helix at the beginning of the N-terminal domain of OCP, which masks the primary site in the C-terminal domain of OCP. We determined the rate of opening of this site and show that it remains exposed long after the relaxation of the red OCP states. Observations of the OCP transitions on the ms time scale revealed that the relaxation of the orange photocycle intermediates is accompanied by an increase in the interaction of the carotenoid keto group with the hydrogen bond donor tyrosine-201. Our data refine the current model of photoinduced OCP transitions and the interaction of its intermediates with FRP.