Regulation of Orange Carotenoid Protein Activity in Cyanobacterial Photoprotection

Abstract

Plants, algae, and cyanobacteria have developed mechanisms to decrease the energy arriving at reaction centers to protect themselves from high irradiance. In cyanobacteria, the photoactive Orange Carotenoid Protein (OCP) and the Fluorescence Recovery Protein are essential elements in this mechanism. Absorption of strong blue-green light by the OCP induces carotenoid and protein conformational changes converting the orange (inactive) OCP into a red (active) OCP. Only the red orange carotenoid protein (OCP r ) is able to bind to phycobilisomes, the cyanobacterial antenna, and to quench excess energy. In this work, we have constructed and characterized several OCP mutants and focused on the role of the OCP N-terminal arm in photoactivation and excitation energy dissipation. The N-terminal arm largely stabilizes the closed orange OCP structure by interacting with its C-terminal domain. This avoids photoactivation at low irradiance. In addition, it slows the OCP detachment from phycobilisomes by hindering fluorescence recovery protein interaction with bound OCP r . This maintains thermal dissipation of excess energy for a longer time. Pro-22, at the beginning of the N-terminal arm, has a key role in the correct positioning of the arm in OCP r , enabling strong OCP binding to phycobilisomes, but is not essential for photoactivation. Our results also show that the opening of the OCP during photoactivation is caused by the movement of the C-terminal domain with respect to the N-terminal domain and the N-terminal arm.Full sunlight is dangerous for plants, algae, and cyanobacteria. It can cause oxidative damages leading to the destruction of the photosynthetic apparatus and to cell death. A short-term photoprotective mechanism developed by oxygenic photosynthetic organisms is the reduction of excitation energy being funneled into the photochemical reaction centers by dissipating excess energy as heat at the level of the antennae (Niyogi and Truong, 2013). In plants and green algae, this mechanism involves the membrane chlorophyll antennae, the light-harvesting complex (for review, see Horton et al., 1996;Horton and Ruban, 2005;Jahns and Holzwarth, 2012), and in cyanobacteria, the extramembrane phycobiliprotein-containing antennae, the phycobilisomes (PBSs; for review, see Kirilovsky and Kerfeld, 2012;Kirilovsky, 2014). Despite these differences in composition and structure of their antennae, carotenoids have an essential role in both plants and cyanobacteria. In plants, high irradiance leads to acidification of the lumen that triggers conformational changes in the light-harvesting complexes and in their organization in the membrane, switching the light-harvesting complex into an effective energy-dissipating form. In cyanobacteria, high irradiance photoactivates a soluble carotenoid protein, the Orange Carotenoid Protein (OCP), that acts as the stress sensor and the energy quencher. In both cases, changes in pigment-pigment interactions (carotenoid-chlorophyll, carotenoid-bilin, chlorophyll-chlorophyll) enable thermal...

Dramatic Domain Rearrangements of the Cyanobacterial Orange Carotenoid Protein upon Photoactivation

“…Our present cross-linking data observed for OCP r add credibility to the hypothesis that OCP undergoes significant conformational changes upon photoactivation, as reached by outcomes from site-directed mutagenesis 11, 15, 46 , protein footprinting 13, 14 , small angle X-ray scattering (SAXS) 13 , and other biophysical techniques 24 . Now the question arises, how do the NTD and CTD re-orient relative to each other in OCP r ?…”

Local and global structural drivers for the photoactivation of the orange carotenoid protein

“…The combination of CD and HDX-MS demonstrates that activation of the OCP results in disruption of the minor interface, the interaction between the N-terminal α-helix and the solvent-exposed β-sheet of the CTD, via the unfolding of the αA (the N-terminal helix). The critical role of the αA helix in OCP photochemistry and function is consistent with a recent mutational analysis that implicated changes in this helix as important to the interaction of the OCP with the PB and the FRP (24). A similar disruption of a helix:domain interaction upon activation was observed in other photoactive proteins, including PYP, BLUF, and LOV (5, 10).…”

A Molecular Mechanism for Nonphotochemical Quenching in Cyanobacteria

“…The N-terminal arm is also a crucial coordinator in the OCP conversion process; this arm not only move away from the CTD but becomes disordered during the activation 12, 42 . Moreover, the absence of the N-terminal arm largely facilitates the action of FRP on OCP r and accelerates the detachment of the OCP r from the PBS 54 . Those previously published findings reinforce our conclusion that the linker-region, N-terminal arm, and CTD on OCP r may be adjacent to the regions that are interacting with FRP.…”