X-ray radiolytic labeling reveals the molecular basis of orange carotenoid protein photoprotection and its interactions with fluorescence recovery protein

• Published in: The Journal of biological chemistry 2019-05, Vol.294 (22)
• Main Authors: Gupta, Sayan, Sutter, Markus, Remesh, Soumya G., Dominguez-Martin, Maria Agustina, Bao, Han, Feng, Xinyu A., Chan, Leanne-Jade G., Petzold, Christopher J., Kerfeld, Cheryl A., Ralston, Corie Y.
• Format: Article
• Language: English
• Subjects: BASIC BIOLOGICAL SCIENCES; carotenoid; carotenoid chromophore; fluorescence recovery protein; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; mass spectrometry (MS); orange carotenoid protein; photosynthetic pigment; protein complex; Protein conformation; time-resolved X-ray footprinting
• ISSN: 0021-9258; 1083-351X

In cyanobacterial photoprotection, the orange carotenoid protein (OCP) is photoactivated under excess light conditions and binds to the light-harvesting antenna, triggering the dissipation of captured light energy. In low light, the OCP relaxes to the native state, a process that is accelerated in the presence of fluorescence recovery protein (FRP). Despite the importance of the OCP in photoprotection, the precise mechanism of photoactivation by this protein is not well understood. Using time-resolved X-ray-mediated in situ hydroxyl radical labeling, we probed real-time solvent accessibility (SA) changes at key OCP residues during photoactivation and relaxation. We observed a biphasic photoactivation process in which carotenoid migration preceded domain dissociation. We also observed a multiphasic relaxation process, with collapsed domain association preceding the final conformational rearrangement of the carotenoid. Using steady-state hydroxyl-radical labeling, we identified sites of interaction between the FRP and OCP. In combination, the findings in this study provide molecular-level insights into the factors driving structural changes during OCP-mediated photoprotection in cyanobacteria, and furnish a basis for understanding the physiological relevance of the FRP- mediated relaxation process.