The purple Trp288Ala mutant of Synechocystis OCP persistently quenches phycobilisome fluorescence and tightly interacts with FRP.

In Cyanobacteria, the Orange Carotenoid Protein (OCP) and Fluorescence Recovery Protein (FRP) are central to the photoprotective mechanism consisting in regulated quenching of phycobilisome (PBs) fluorescence. Due to a transient and flexible nature of the light-activated red quenching form, OCP, which is obtained from the stable dark-adapted orange form, OCP, by photoconversion, the detailed mechanism of photoprotection remains unclear. Here we demonstrate that our recently described W288A mutant of the Synechocystis OCP (hereinafter called OCP) is a fully functional analogue of the OCP form which is capable of constitutive PBs fluorescence quenching in vitro with no need of photoactivation. This PBs quenching effect is abolished in the presence of FRP, which interacts with OCP with micromolar affinity and an apparent stoichiometry of 1:1, unexpectedly, implying dissociation of the FRP dimers. This establishes OCP as a robust model system providing novel insights into the interplay between OCP and FRP to regulate photoprotection in cyanobacteria.