Charge reversal at the Lhcb2 N-terminus impairs phosphorylation and PSI-LHCII complex formation

Abstract

State transitions balance excitation-energy distribution between Photosystem I and Photosystem II in higher plants. Stn7-mediated phosphorylation of the N-terminus of the light-harvesting complex II protein Lhcb2 plays a central role in photosynthetic state transitions. However, it remains unclear how the intrinsic charge of this region, independent of its phosphorylation status, influences state transitions and thylakoid membrane organization. Here, we introduced specific charge-altering mutations in the Lhcb2 N-terminus of Arabidopsis thaliana in the lhcb2 knock-out background and analyzed their effects on LHCII phosphorylation, state transition dynamics, PSI-LHCII complex formation, and thylakoid ultrastructure. Substitution of a conserved positively charged arginine with a negatively charged glutamate (R2E) markedly reduced Lhcb1 and Lhcb2 phosphorylation and state transition efficiency, and abolished PSI-LHCII complex formation. In contrast, introducing a negative charge at a downstream position (Q9E) had no detectable effects. Electron microscopy revealed no significant changes in thylakoid organization in either mutant compared to WT Lhcb2 plants. Despite strongly reduced Lhcb1 and Lhcb2 phosphorylation in the R2E mutant, residual state transitions persisted, potentially mediated by Stn7-dependent phosphorylation of other target proteins. Together, these results provide insight into the role of N-terminal LHCII electrostatics in state transitions and thylakoid membrane organization in plants.