SOQ1 functions as a methionine sulfoxide reductase in the chloroplast lumen for regulation of photoprotective qH in Arabidopsis

Photosynthetic organisms must balance light absorption and energy dissipation to prevent photo-oxidative damage. Non-photochemical quenching (NPQ) dissipates excess light energy as heat, with the quenching component qH providing sustained photoprotection. However, the molecular mechanism underlying qH induction remains unclear. Our study focuses on the thylakoid membrane protein SUPPRESSOR OF QUENCHING 1 (SOQ1) and its inhibition of qH through interaction with LIPOCALIN IN THE PLASTID (LCNP) in Arabidopsis thaliana. Structural homology of SOQ1 lumenal domains with bacterial disulfide bond protein D suggested potential thiol-disulfide exchange activity. In vitro assays determined that both SOQ1 thioredoxin-like (Trx-like) and C-terminal (CTD) domains contain a redox-active cysteine pair and evidenced electron transfer from Trx-like to CTD. Importantly, we found that SOQ1 lumenal domains exhibit methionine sulfoxide reductase (Msr) activity converting oxidized methionine residues in LCNP back to methionine, which thereby inactivates LCNP and prevents qH formation. Mutational analyses identified cysteine residues in SOQ1-CTD and methionine residues in LCNP as critical for qH suppression, supporting their role in redox regulation. Additionally, we found that the redox state of SOQ1 in vivo is light-dependent, shifting from reduced to oxidized under stress conditions, indicating a dynamic regulation of its activity. We conclude that the Trx-like domain of SOQ1 provides reducing power to its CTD displaying Msr activity. SOQ1 is therefore an unusual example of a protein possessing both a disulfide reductase and Msr domain in tandem. Our findings elucidate the redox-regulation mechanism of qH involving SOQ1-mediated methionine reduction of LCNP, providing insights into the intricate control of photoprotective processes in chloroplasts and enhancing our understanding of plant resilience under environmental stress.