Prokaryotic Deg (HtrA) proteases are involved in protein quality control and response to stress [1]. The
Arabidopsis thaliana genome contains 16 Deg genes whose products are distributed in chloroplasts, mitochondria, peroxisomes and the nucleus [2]. Deg2 and Deg7 are located in the chloroplast stroma, whereas Deg1, Deg5 and Deg8 are found in the thylakoid lumen. Deg1 forms active homo-hexamers at acidic pH, degrading photosynthetic proteins, especially in relation to the PSII repair cycle [3,4]. Deg5 and Deg8 form hetero-complexes, performing apparently similar functions [5], raising the question whether the two complexes are redundant. To answer this, we generated a full set of single, double and triple KO mutants and compared their phenotypes. We found that under optimal growth conditions Deg5-Deg8 mutants look like WT, but Deg1 mutants are smaller and show higher sensitivity to photoinhibition. Under harsher conditions, Deg5-Deg8 mutants are also affected, although less than Deg1 mutants. However, the functions of the two complexes are somewhat redundant, as overexpression of Deg5-Deg8 can partially compensate for the loss of Deg1. Comparative proteomics revealed in the triple mutant moderate up-regulation of thylakoid proteins involved in folding, translocation, assembly and degradation, and down-regulation of components of all photosynthetic complexes. Testing the steady-state level of the thylakoid Deg proteases in WT plants demonstrated that Deg1 is approximately two-fold more abundant than the Deg5-Deg8 complex. Moreover, recombinant Deg1 had higher
in vitro proteolytic activity compared with Deg5, Deg8 and the combination of the two. These results suggest that the differences in abundance and proteolytic activity are the source of the differential importance of the two complexes
in vivo.
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