Abstract
In plants oxygenic photosynthesis is performed by large protein complexes found in the thylakoid membranes of chloroplasts. The soluble thylakoid lumen space is a narrow and compressed region within the thylakoid membrane which contains 80–200 proteins. Because the thylakoid lumen proteins are in close proximity to the protein complexes of photosynthesis, it is reasonable to assume that the lumen proteins are highly influenced by the presence of light.To identify light regulated proteins in the thylakoid lumen of Arabidopsis thaliana we developed a faster thylakoid preparation and combined this with difference gel electrophoresis (DIGE) of dark-adapted and light-adapted lumen proteomes. The DIGE experiments revealed that 19 lumen proteins exhibit increased relative protein levels after eight hour light exposure. Among the proteins showing increased abundance were the PsbP and PsbQ subunits of Photosystem II, major plastocyanin and several other proteins of known or unknown function. In addition, co-expression analysis of publicly available transcriptomic data showed that the co-regulation of lumen protein expression is not limited to light but rather that lumen protein genes exhibit a high uniformity of expression.The large proportion of thylakoid lumen proteins displaying increased abundance in light-adapted plants, taken together with the observed uniform regulation of transcription, implies that the majority of thylakoid lumen proteins have functions that are related to photosynthetic activity. This is the first time that an analysis of the differences in protein level during a normal day/night cycle has been performed and it shows that even a normal cycle of light significantly influences the thylakoid lumen proteome. In this study we also show for the first time, using co-expression analysis, that the prevalent lumenal chloroplast proteins are very similarly regulated at the level of transcription.
Highlights
Higher plants, algae and cyanobacteria have the ability to perform oxygenic photosynthesis, and in higher plants the process is centred in the thylakoid membrane of the chloroplast
In this work we show, using difference gel electrophoresis (DIGE) on fast lumen preparations, that 21 proteins changed in abundance between 8 h-light and 16 h-dark acclimated Arabidopsis plants
Analysis of data from Arabidopsis seedlings, grown in 8 h light/16 h dark cycles under comparable growth conditions as used in this study shows that mRNA expression for the majority of prevalent lumen proteins cycles in a diurnal manner, with expressions peaking during the light period
Summary
Algae and cyanobacteria have the ability to perform oxygenic photosynthesis, and in higher plants the process is centred in the thylakoid membrane of the chloroplast. Considering the obvious, being the location of the lumen proteins within the thylakoid membrane, the heart of photosynthesis, it would be expected that several if not many of the proteins participate in the regulation of photosynthesis. Among those lumen proteins which have not previously been identified as extrinsic PSII components several have been implicated in PSII function. PPL1, a member of the PsbP-like protein family, as well as the 18.3 kDa lumen protein, have both recently been shown to play roles in the efficient repair of photodamaged PSII [4,5]. Mutant plants containing a T-DNA insertion in the CYP38 gene are defect in assembly of PSII supercomplexes [6]
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