The molecular origin of the OCP-dependent non-photochemical quenching mechanism in cyanobacteria

Cyanobacteria were the first microorganisms that released oxygen into the atmosphere billions of years ago. To do it safely under intense sunlight, they developed strategies that prevent photooxidation in the photosynthetic membrane. One of these strategies is accomplished by regulating the light-harvesting activity of their antenna complexes – the phycobilisomes – via the orange-carotenoid protein (OCP). This water-soluble protein encapsulates a ketocarotenoid and is photoactive. Under strong irradiance, OCP interacts with the phycobilisomes and triggers non-photochemical quenching (NPQ), a mechanism that safely dissipates overexcitation in the membrane. To date, the mechanism of action of OCP in triggering NPQ is unknown. We here applied ultrafast spectroscopy on the active domain of OCP bound to the phycobilisome core. Our results demonstrate that the binding to the phycobilisomes modifies the structure of the ketocarotenoid. We show that this molecular switch activates NPQ, by enabling energy transfer from the antenna pigments to the ketocarotenoid.