- © 2011 American Society of Plant Biologists
Photosystem II (PSII), the primary reaction center of the light reactions of photosynthesis, is a well-studied complex containing more than 20 different subunits and associating with over 100 other accessory light-harvesting proteins and cofactors. PSII requires continual maintenance and repair, as excess energy from the light reactions generates damaging reactive oxygen species. The D1 reaction center protein undergoes a particularly high level of turnover, especially under high light conditions. More than 25 auxiliary proteins have been reported to be involved in the repair cycle of PSII (Mulo et al., 2008), yet new proteins functioning in this process continue to be discovered, and there are numerous thylakoid proteins of unknown function (Ajjawi et al., 2010; Lu et al., 2011b).
Lu et al. (2011a; pages 1861–1875) characterize two low quantum yield of PSII 1 (lqy1) mutants and identify LQY1 as a small Zn-finger thylakoid membrane protein involved in PSII maintenance. The authors isolated lqy1 mutants from a screen of 5000 T-DNA mutants (Lu et al., 2011b) based on their reduced photosynthetic competence following high light treatment (determined by measuring chlorophyll fluorescence parameters). The lqy1 mutants were found to have lower maximum PSII photochemical efficiency (Fv/Fm) and higher nonphotochemical quenching than wild-type plants as well as elevated accumulation of reactive oxygen species following high light treatment (see figure). After high light treatment, mutant plants also had less PSII–light-harvesting complex II supercomplex than wild-type plants, indicating a defect in the repair mechanism under high light. Consistent with the hypothesis that it has a role in repair, LQY1 protein was found to comigrate with PSII core monomer in thylakoid membrane preparations.
Enhanced accumulation of reactive oxygen species in lqy1 mutants. Nitro blue tetrazolium staining of O2− in Col wild type (left), lqy1-1 (middle), and lqy1-2 (right) plants after growing under high light for 2 d. (Figure reprinted from Figure 3 of Lu et al. [2011a].)
LQY1 has sequence similarity to DnaJ Zn-finger molecular chaperone proteins, which have disulfide isomerase activity associated with the Zn-finger domain that maintains target protein disulfide pairs in a reduced and enzymatically active conformation. Enzymatic assays showed that LQY1 has disulfide isomerase activity capable of reactivating reduced and denatured RNase A, similar to DnaJ from Escherichia coli. A number of the PSII core proteins, including D1, D2, CP47, and CP43, have two or more Cys residues and are possible substrates for LQY1. Interestingly, the absence of LQY1 was associated with higher turnover and synthesis of D1 protein. This is consistent with the hypothesis that LQY1 assists with maintenance and repair of photodamaged D1, as a decline in the repair mechanism would lead to accumulation of excess photodamaged D1, in turn triggering higher rates of degradation and higher rates of biosynthesis to replace the damaged protein.
LQY1 homologs were found in the genomes of numerous land plants, including angiosperms, gymnosperms, the bryophyte moss Physcomitrella, and the lycopod Selaginella, but not in cyanobacteria or algae, suggesting broad conservation of function in land plants. The recent characterization of LQY1 and other PSII auxiliary proteins in PSII maintenance and repair indicates that, despite years of study of PSII, there are still additional proteins and regulatory mechanisms to be identified.
