Structural insights into PsbS and its interactions with the PSII-LHCII supercomplex under active and inactive nonphotochemical states in Spinacia oleracea

Abstract

Nonphotochemical quenching (NPQ) is a photoprotective mechanism activated under strong light conditions to safely dissipate excess light energy into heat. This mitigates the production of radical oxygen species (ROS) and the photooxidative damage of thylakoidal proteins. The PsbS protein is a key component of NPQ in higher order plant photosynthesis. Higher plants’ thylakoid membranes are characterized for being enriched in Photosystem II (PSII) and light-harvesting complex II (LHCII) which together form the PSII-LHCII supercomplex (PSII-LHCII sc) and form stacked structures in the grana. Under excess light conditions, the thylakoid lumen acidifies activating PsbS which mediates light-regulated interactions in the PSII-LHCII sc leading to its macroorganization and causing structural changes in the grana stacks. The light-dependent conformational changes of PsbS and its interactions with PSII and LHCII components are still largely unknow. Additionally, it has been shown that altering NPQ could have significant implications in crop engineering to increase plant productivity. This makes understanding the dynamic behavior of PsbS and the photosynthetic structures it interacts with of great interest. In this project, we aim to extract structural details of the PsbS conformation and its interactions with the PSIILHCII sc under NPQ-inactive (dark) and NPQ-active (light) states. To achieve this, we make use of crosslinking mass spectrometry (XL-MS) structural proteomics approaches and molecular modelling. We also explore the incorporation of structural evolutionary predictions, via AlphaFold 2 and Evolutionary couplings, to complement the experimental data. Our results show that we find dimeric PsbS under both dark and light conditions with different conformations in its N-termini. Moreover, we use the XL-MS data to build a structural model which shows that PsbS, Lhcb6 and the LHCII moderately bound trimers disassociate together from the PSII-LHCII sc under NPQ conditions.