Protein/linker dynamics facilitates the structural adaptation of the orange carotenoid protein in cyanobacterial photoprotection: a neutron scattering study.

The orange carotenoid protein (OCP) plays a pivotal role in the photoprotection of cyanobacteria and undergoes a structural change after photoactivation. Here, we report a solution structure of the active state differing significantly from that known upon binding to the phycobilisome. In solution, the N-terminal domains are clearly detached and reoriented by about 90° relative to the C- terminal domains. This apparent structural difference implies the need for a previously unknown structural rearrangement. A rarely considered aspect is the internal protein dynamics facilitating this structural transition. Therefore, we directly probe protein dynamics in a wide observation time range in the dark and under illumination. After photoactivation to the active state, the internal dynamics is enhanced, while the global diffusion is slowed down. A residue-resolved molecular dynamics simulation indicates that the interdomain linker becomes more flexible and may mediate the structural adaptation to the phycobilisome binding site. In turn, the β-sheet structures remain the most rigid part of the active state structure, which is essential for protein dimerization.