Teamwork of DEG Proteases in Protein Degradation on Both Sides of the Thylakoid Membrane
Final Report Abstract
Life is characterised by constant turnover and renewal of all its components. Proteins, as the main executors of biochemical tasks, especially need to be kept in an active and ordered state. Therefore, plants and all other organisms have an elaborate system of proteostasis, in which biosynthesis, folding, activity and degradation of proteins are coordinately regulated to keep the cellular proteome functional. DEG proteases are a class of ATP-independent serine proteases with endoproteolytic activity and are predominantly targeted to membrane-enclosed organelles in plant cells. We used reverse genetics, biochemical and physiological approaches to characterise seven members of the DEG protease family in the model plant Arabidopsis. Within the chloroplast, the stromal protease DEG2 and the lumenal proteases DEG1, 5 and 8 cooperate with the FTSH protease to ensure removal of photo-damaged proteins, especially the photosystem reaction centre protein D1. We demonstrated that there is crosstalk between DEG2 and DEG1 across the thylakoid membrane. In the thylakoid lumen, the role of DEG5/8 hetero-oligomers is especially evident under light stress conditions, when the capacity of DEG1, which provides the major part of the proteolytic activity, may become limiting for the repair of damaged photosystems. The abundance of at least 316 plastid-localised proteins was changed under light stress conditions, when either DEG1 alone or all three lumenal DEG proteases were deleted. DEG1 seems to have additional, unique tasks, because overexpression of DEG5 and 8 could only partially compensate the lack of DEG1 in mutant plants. Also the cell nucleus contains two DEG proteases. While DEG7 in the nucleoplasma seems to regulate programmed cell death, the function of the nucleolar protease DEG9 remained elusive. Mutants lacking DEG9 showed no detectable defects in the processing of ribosomal RNAs or the functionality of ribosomes, indicating that DEG9 does not contribute to the main function of the nucleolus, the biogenesis of ribosomes. According to prediction programs, several DEG proteases are localised in mitochondria. We demonstrated that DEG10 is predominantly expressed in mitochondria of leaf hairs (trichomes) and in the cells of the vascular tissue. The activity of DEG10 is required for sustained root growth during high temperature stress and for optimal seed production under challenging climatic conditions in the field. As an emerging conclusion, the activity of DEG proteases seems to be especially important under stressful conditions in plants. Apparently they are essential for the maintenance of proteostasis under stress conditions when cellular energy levels are low and ATP-dependent proteases fail to remove all damaged or unwanted proteins. Therefore, modulation or engineering of DEG protease activity provides a promising strategy to increase stress tolerance of plants, which could help to alleviate the risk of food shortage induced by climate change.
Publications
- (2012) DEG Proteases in Arabidopsis thaliana, PhD thesis, University of Konstanz, pp 108
Erhardt M
- (2014) Biochemical and physiological characterisation of Deg/HtrA proteases in Synechocystis sp. PCC 6803, PhD thesis, University of Konstanz, pp 106
Perthold M
- (2014) Lokalisierung und physiologische Charakterisierung der Serinprotease DEG7 in Arabidopsis thaliana, PhD thesis, PhD thesis, University of Konstanz, pp 115
Mogg U
- (2015) The DEG9 and DEG14 proteases in A. thaliana, PhD thesis, University of Konstanz, pp 124
Gasparic, K
