PT  - JOURNAL ARTICLE
AU  - Nikolai N. Sluchanko
AU  - Yury B. Slonimskiy
AU  - Marcus Moldenhauer
AU  - Thomas Friedrich
AU  - Eugene G. Maksimov
TI  - Deletion of the short N-terminal extension in OCP reveals the main site for the FRP binding
AID  - 10.1101/116491
DP  - 2017 Jan 01
TA  - bioRxiv
PG  - 116491
4099  - http://biorxiv.org/content/early/2017/03/13/116491.short
4100  - http://biorxiv.org/content/early/2017/03/13/116491.full
AB  - The photoactive Orange Carotenoid Protein (OCP) plays a central role in cyanobacterial photoprotection. Photoconversion entails significant structural rearrangements in OCP required for its binding to the phycobilisome to induce excitation energy dissipation, whereas the fluorescence recovery protein (FRP) is required for OCP detachment and restoration of phycobilisome fluorescence. Although key to understanding the whole reversible mechanism of photoprotection, the FRP binding site on OCP has been representing challenge since the discovery of FRP in 2010 and is currently unknown. OCP comprises two structural domains organized into a compact basic orange form due to specific protein-chromophore and inter-domain protein-protein interactions and interacts with FRP tightly only when photoactivated. As an important stabilizing element in the orange OCP, the short αA-helix within the N-terminal extension (NTE) binds to OCP’s C-terminal domain (CTD), but unfolds upon photoactivation and interferes with phycobilisome binding. By using an alloy of biochemical and biophysical techniques, 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.