DEG Proteases in Arabidopsis thaliana

Abstract

Proteases are catalytic enzymes, not only responsible for the degradation of unwanted proteins but also representing important regulatory factors in planta. The selective cleavage of peptide bonds marks crucial post-translational modifications for proper protein maturation. But not only protein maturation is regulated by proteases, selective activation or deactivation of whole molecular response mechanism can be attributed to them. Therefore, it is rather unsurprising that 3 % of all, genes in the Arabidopsis thaliana genome are encoding for proteases. The family of DEG proteases thereby represents a small group of 16 ATP-independent serine-type proteases in A. thaliana. With the present thesis, the physiological role of selected DEG proteases was investigated. We were able to confirm previously predicted localisations for various DEG proteases and presented hypotheses about their function in the respective organelles. Nuclear DEG proteases were especially highlighted and the plant cell nucleus pictured as the main cellular regulator. However, the main focus of this thesis was laid on the chloroplastic protease DEG2. Initially characterised as an important factor in light stress response mechanism, specifically cleaving the D1 protein of photosystem II (PSII), the function of DEG2 became more questionable over the following years. This thesis presents new data arising from comparison of A. thaliana wild-type (wt) plants with knock-out (ko) mutants lacking DEG2 (deg2), which is present mainly in photosynthetically active tissue. It is demonstrated that DEG2 protein level remains constant in response to induced lightstress, questioning the involvement in photorepair mechanism. However, comparative proteomics revealed the up and down-regulation of several subunits of the photosynthetic apparatus, suggesting the involvement of DEG2 in a superordinate regulation network in the plant chloroplast. Biochemical approaches revealed the formation of a trimeric DEG2 homo-oligomer in vivo, while hexamers were observed in vitro. Phenotypical comparison of A. thaliana wt plants with deg2 ko mutants showed that the latter possess a shorter root length under in vitro conditions. We were able to complement this striking phenotype by addition of sucrose to the growth medium, indicating a possible involvement of DEG2 in carbon metabolism. In summary, our data implies that DEG2 is essential for normal plant development and does possess a much broader substrate specificity than previously described.