Quantifying the Acidification-Induced Shift of the Dimerization Equilibrium of PsbS.

Plants protect themselves from excess light by activating NPQ, a process triggered in the thylakoid by the pH-sensitive membrane protein PsbS upon lumen acidification and protonation of lumen-exposed glutamates. However, how these protonation events are coupled to PsbS oligomerization and photoprotective function is not fully understood. The present study combines constant-pH molecular dynamics simulations of monomeric and dimeric PsbS with a thermodynamic linkage analysis, providing the first direct quantification of the pH-induced shift on the dimerization free energy and monomer fraction. Results indicate only a small acidification-induced shift of the equilibrium toward the monomer under physiological conditions. Analysis of residue-level free-energy contributions reveals key pH-sensitive residues previously found as functionally important in experiments. We further identify complex, pH-dependent protonation correlation networks, especially under acidic conditions, and long-range correlations across the membrane, consistent with communication between lumenal and stromal regions in both oligomeric states.