The 35 kDa water-soluble Orange Carotenoid Protein (OCP) is responsible for photoprotection in cyanobacteria. It acts as a light intensity sensor that simultaneously serves as efficient quencher of phycobilisome excitation energy as well as of reactive oxygen species. Photoactivation triggers large-scale conformational rearrangements to convert OCP from the orange OCPO state to the red active signaling state OCPR, as demonstrated by various structural methods. Eventually, such rearrangements imply complete yet reversible separation of structural domains (C- and N-terminal domain) and significant translocation of the carotenoid cofactor. Very recently, dynamic crystallography of OCPO crystals suggested the existence of photocycle intermediates with small-scale rearrangements that may trigger further transitions in the protein. However, the currently existing gap between the ultra-fast picosecond and 100 millisecond time scale of spectroscopic and structural data precludes knowledge about distinct intermediate states. In this study, we took advantage of single 7 ns laser pulses to study carotenoid absorption transients in OCP on the time-scale from 100 ns to 10 s, which allowed us to detect a red intermediate state preceding the red signaling state OCPR. In addition, time-resolved fluorescence spectroscopy and following assignment of carotenoid-induced quenching of different tryptophan residues revealed a novel orange intermediate state, which appears during back-relaxation of photoactivated OCPR to OCPO. Our results show asynchronous changes in the carotenoid and protein components and provide refined mechanistic information about the OCP photocycle as well as introduce new kinetic signatures for future studies of OCP photoactivity and photoprotection.

The photocycle of orange carotenoid protein conceals distinct intermediates and asynchronous changes in the carotenoid and protein components