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