Photoprotective mechanisms are of fundamental importance for the survival of photosynthetic organisms. In cyanobacteria, the orange carotenoid protein (OCP), when activated by intense blue light, binds to the light-harvesting antenna and triggers the dissipation of excess captured light energy. Using a combination of multiple techniques including X-ray footprinting, Gupta et al identified both the local and global structural changes in the OCP upon photoactivation. Microsecond radiolytic labeling identified rearrangement of the H-bonding network associated with conserved residues and structural water molecules. Collectively, these data provide experimental evidence for an ensemble of local and global structural changes, upon activation of the OCP, that are essential for photoprotection.
Figure 4 from Gupta et. al: Water network for carotenoid signaling in OCP and proposed signal propagation pathway through the water-side chain H-bonding network to the protein surface that facilitates shift, disssociation of NTD-CTD, and detachment of the N-terminal helix from CTD.
Results from: Gupta S, Guttman M, Leverenz RL, Zhumadilova K, Pawlowski EG, Petzold CJ, Lee KK, Ralston CY and Kerfeld CA. Local and global structural driver for the photoactivation of the orange carotenoid protein. Proc Natl Acad Sci U S A. 2015 Oct 13; 112(41):E5567-74. PMCID: PMC4611662