| Popis: |
The photosystem II PsbS protein triggers the photo-protective mechanism of plants – nonphotochemical quenching (NPQ) – by sensing the acidification of the thylakoid lumen. A popular model of the NPQ activation involves the monomerization of the PsbS dimer. However, a clear connection between the PsbS pH-dependent structural changes and the dimer stability is still missing. Here, we apply a multiscale computational workflow including constant pH coarse grained (CG) and all-atom (AA) molecular dynamics simulations to investigate the structural response of the PsbS dimer to the pH variation. We find that most of the Glu residues undergo significant pKa shifts, in line with previous predictions for the monomeric state. Protonation of the key Glu173 residue induces the movement of an amphipathic helix, located at the dimeric interface, from the membrane to the aqueous environment, providing the first atomistic model of the PsbS dimer in its neutral pH conformation. Moreover, free energy profiles for the monomerization process, obtained with metadynamics simulations at the CG level, reveal that in the neutral pH conformation, where the network of H-bond interactions at the dimeric interface is destroyed, the protein-protein interaction is weaker. Taken together, our results show how the pH-dependent conformations of PsbS affect their dimerization propensity, which is at the basis of the NPQ mechanism. |
