Changes in specific protein degradation rates inArabidopsis thalianareveal multiple roles of Lon1 in mitochondrial protein homeostasis

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Mitochondrial Lon1 loss impairs oxidative phosphorylation complexes and TCA enzymes and causes accumulation of specific mitochondrial proteins. Analysis of over 400 mitochondrial protein degradation rates using 15N labelling showed that 205 were significantly different between wild type (WT) andlon1-1. Those proteins included ribosomal proteins, electron transport chain subunits and TCA enzymes. For respiratory complexes I and V, decreased protein abundance correlated with higher degradation rate of subunits in total mitochondrial extracts. After blue native separation, however, the assembled complexes had slow degradation, while smaller subcomplexes displayed rapid degradation inlon1-1. In insoluble fractions, a number of TCA enzymes were more abundant but the proteins degraded slowly inlon1-1. In soluble protein fractions, TCA enzymes were less abundant but degraded more rapidly. These observations are consistent with the reported roles of Lon1 as a chaperone aiding the proper folding of newly synthesized/imported proteins to stabilise them and as a protease to degrade mitochondrial protein aggregates. HSP70, prohibitin and enzymes of photorespiration accumulated inlon1-1and degraded slowly in all fractions, indicating an important role of Lon1 in their clearance from the proteome.

Significance Statement

Lon1 is a mitochondrial chaperone that aids proper folding and thus stabilizes newly synthesized/imported proteins, but it is also a protease that degrades mitochondrial protein aggregates. It is unknown how loss of a specific protease affects organelle protein degradation rates in plants. Here we used the lon1 mutant and in vivo15N labelling of proteins in hydroponically grown plants to determine the impact of lon1 absence on organelle protein degradation rates.

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