Dimerization of Immunophilin CYN38 Regulates Photosystem II Repair In Chlamydomonas

The high light (HL) tolerance of Chlamydomonas determines biomass productivity under excess light conditions. The repair cycle of photosystem II (PSII) is a fundamental process that ensures long-term HL adaptation in photosynthetic organisms. Immunophilins, originating from cyanobacteria and surged in eukaryotic photosynthetic species, were characterized to play pivotal functions for HL adaptation by influencing PSII activity directly or indirectly. Here, we identified that Chlamydomonas immunophilin CYN38, the conserved homolog of Arabidopsis CYP38, was localized in the thylakoid lumen. One intriguing cyn38 mutant caused by the insertion mutation to produce a longer protein CYN38(L) with an extended C terminus was characterized. The cyn38 mutant displayed HL sensitive phenotype, with dramatically reduced accumulation of PSII supercomplexes and PSII core subunits under HL treatment. In WT, CYN38 forms a homodimer relying on its C terminus and associates with PSII complexes. In cyn38, the CYN38(L) protein can neither dimerize nor associate with PSII complexes, which causes defective PSII repair. Taken together, our work demonstrated the conserved physiological function of CYN38 during PSII biogenesis in photosynthetic species and unraveled a previously unidentified dimerization of CYN38 for its function in PSII repair under HL stress.