A Glimpse into the Sequence of Structural Changes in the Orange Carotenoid Protein Which Switch on the Photoprotection Mechanism in Cyanobacteria

Abstract

Photoprotective mechanisms are key to the survival of photosynthetic organisms. In cyanobacteria, the Orange Carotenoid Protein (OCP) senses excess light and binds to the light-harvesting antenna, triggering the dissipation of captured light energy. Using time-resolved X-ray footprinting with mass spectrometry (XFMS), we traced the time-evolution of both the local and global structural changes in the OCP photoactivation and relaxation processes. Activation is accompanied by a fast decrease in the solvent accessibility around conserved carotenoid binding residues, and a slow increase in the solvent accessibility at the inter-domain interface. Accessibility changes in the relaxation process follow a reverse trend and, in addition, pass through a compact intermediate state. This study identified the driving factors for structural changes transmitted from the carotenoid binding pocket to the protein surface upon light activation of the OCP as well as the back-conversion process with unprecedented mechanistic detail. The physiological relevance of the kinetic mechanism is explained in terms of the mode of interaction between the Fluorescence Recovery Protein (FRP) and the OCP.