A Novel Non-photochemical Quenching Mechanism of LHCII at High-temperature

Major light-harvesting complex of Photosystem II (LHCII) carries a photoprotective function of non-photochemical quenching (NPQ), which is triggered by acidity and elevated temperature, safely dissipating the excess excited energy as heat. Elucidating the molecular basis of thermally induced NPQ has become increasingly urgent as global warming progresses. NPQ is regarded as being regulated by reversible aggregation/de-aggregation of LHCII, the quenching mechanism is attributed to the energy transfer from chlorophyll (Chl) a Qy to lutein 1 (Lut1) S1 state. Here we resolved two types of LHCII cryo-electron microscopy structures with significant protein secondary structural changes at 40 and 55 °C, one with Lut1 structure remaining undistorted, the other with distorted Lut1 having a shorter Lut1-Chl612 distance, resulting a quenching state, supported by molecular dynamics simulation. Femtosecond stimulated Raman spectroscopy directly confirmed the energy transfer from Chl to Lut via the vibrational fingerprint of Lut S1 state. In addition to high-temperature induced NPQ pathway above 43 °C requiring LHCII-aggregation, we found a novel thermally triggered and aggregation-independent quenching pathway, which involves Lut1 distortion above room temperature.