A Small Zinc Finger Thylakoid Protein Plays a Role in Maintenance of Photosystem II in Arabidopsis thaliana

Yan Lu,Robert L Last,David A Hall

doi:10.1105/tpc.111.085456

Abstract

This work identifies LOW QUANTUM YIELD OF PHOTOSYSTEM II1 (LQY1), a Zn finger protein that shows disulfide isomerase activity, interacts with the photosystem II (PSII) core complex, and may act in repair of photodamaged PSII complexes. Two mutants of an unannotated small Zn finger containing a thylakoid membrane protein of Arabidopsis thaliana (At1g75690; LQY1) were found to have a lower quantum yield of PSII photochemistry and reduced PSII electron transport rate following high-light treatment. The mutants dissipate more excess excitation energy via nonphotochemical pathways than wild type, and they also display elevated accumulation of reactive oxygen species under high light. After high-light treatment, the mutants have less PSII-light-harvesting complex II supercomplex than wild-type plants. Analysis of thylakoid membrane protein complexes showed that wild-type LQY1 protein comigrates with the PSII core monomer and the CP43-less PSII monomer (a marker for ongoing PSII repair and reassembly). PSII repair and reassembly involve the breakage and formation of disulfide bonds among PSII proteins. Interestingly, the recombinant LQY1 protein demonstrates a protein disulfide isomerase activity. LQY1 is more abundant in stroma-exposed thylakoids, where key steps of PSII repair and reassembly take place. The absence of the LQY1 protein accelerates turnover and synthesis of PSII reaction center protein D1. These results suggest that the LQY1 protein may be involved in maintaining PSII activity under high light by regulating repair and reassembly of PSII complexes.

Highlights

Harnessing the tremendous energy of photons by photosynthesis is both essential and risky for green plants; this energy powers plant metabolism but can damage the photosynthetic apparatus
The results presented in this article are consistent with the hypothesis that LOW QUANTUM YIELD OF PHOTOSYSTEM II1 (LQY1) is involved in the repair and reassembly cycle of the photosystem II (PSII)–light-harvesting complex II (LHCII) supercomplex under high irradiance
As part of a reverse genetics project designed to discover novel functions for chloroplast-targeted proteins, ;5200 Arabidopsis homozygous T-DNA lines were analyzed for alterations in PSII photosynthetic electron transport (Lu and Last, 2008; Lu et al, 2008; Ajjawi et al, 2010)

Summary

Introduction

Harnessing the tremendous energy of photons by photosynthesis is both essential and risky for green plants; this energy powers plant metabolism but can damage the photosynthetic apparatus. Despite the importance of carbon fixation in agriculture, key mechanistic details remain to be elucidated for some aspects of photosynthesis, such as energy dissipation, cyclic electron transport, and formation, repair, and degradation of the photosynthetic apparatus (Baena-Gonzalez and Aro, 2002; Tanaka and Makino, 2009). The continued identification of new proteins associated with PSII indicates that there is more to learn about its structure and function. This is challenging for a variety of reasons, including low protein abundance, labile interactions with core and antenna complexes, and functional redundancy (Mulo et al, 2008). In an attempt to identify functions for novel chloroplast proteins, thousands of Arabidopsis thaliana T-DNA mutants with insertions in nuclear genes for chloroplasttargeted proteins are being analyzed for a variety of mutant phenotypes, including chlorophyll fluorescence (Lu et al, 2008, 2011; Ajjawi et al, 2010)

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