TLP18.3, a novel thylakoid lumen protein regulating photosystem II repair cycle

A proteome analysis of Arabidopsis thaliana thylakoid-associated polysome nascent chain complexes was performed to find novel proteins involved in the biogenesis, maintenance and turnover of thylakoid protein complexes, in particular the PSII (photosystem II) complex, which exhibits a high turnover rate. Four unknown proteins were identified, of which TLP18.3 (thylakoid lumen protein of 18.3 kDa) was selected for further analysis. The Arabidopsis mutants (SALK_109618 and GABI-Kat 459D12) lacking the TLP18.3 protein showed higher susceptibility of PSII to photoinhibition. The increased susceptibility of ΔTLP18.3 plants to high light probably originates from an inefficient reassembly of PSII monomers into dimers in the grana stacks, as well as from an impaired turnover of the D1 protein in stroma exposed thylakoids. Such dual function of the TLP18.3 protein is in accordance with its even distribution between the grana and stroma thylakoids. Notably, the lack of the TLP18.3 protein does not lead to a severe collapse of the PSII complexes, suggesting a redundancy of proteins assisting these particular repair steps to assure functional PSII. The ΔTLP18.3 plants showed no clear visual phenotype under standard growth conditions, but when challenged by fluctuating light during growth, the retarded growth of ΔTLP18.3 plants was evident. Abbreviations: BN, blue native; Chl, chlorophyll; cyt, cytochrome; 2D, two-dimensional; DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea; DMBQ, 2,6-dimethyl-p-benzoquinone; DUF, domain of unknown function; FNR, ferredoxin–NADP+ reductase; GL, growth light; HL, high light; IEF, isoelectric focusing; LHC, light-harvesting complex; MALDI–TOF, matrix-assisted laser-desorption-ionization time-of-flight; OEC, oxygen evolving complex; PAM, pulse amplitude modulation; PSI, photosystem I; PSII, photosystem II; T-DNA, transfer DNA; TMH, transmembrane helix; TLP18.3, thylakoid lumen protein of 18.3 kDa; TLP18.3*, truncated form of TLP18.3; WT, wild-type