Deletion of the short N-terminal extension in OCP reveals the main site for FRP binding.

The orange carotenoid protein (OCP) plays a key role in cyanobacterial photoprotection. Photoconversion entails structural rearrangements in OCP that are required for its binding to phycobilisome, thereby inducing excitation energy dissipation. Detachment of OCP from phycobilisome requires the fluorescence recovery protein (FRP). It is considered that OCP interacts with FRP only in the photoactivated state; however, the binding site for FRP is currently unknown. As an important stabilizing element in orange OCP, the short αA-helix within the N-terminal extension (NTE) binds to the C-terminal domain (CTD), but unfolds upon photoactivation and interferes with phycobilisome binding. Here, we demonstrate that the NTE shares specific structural and functional similarities with FRP and discover the main site of OCP-FRP interactions in the OCP-CTD.