Deg proteases and their role in protein quality control and processing in different subcellular compartments of the plant cell

Abstract

Degradation of periplasmic proteins (Deg)/high temperature requirement A (HtrA) proteases are ATP-independent serine endopeptidases found in almost every organism. Database searches revealed that 16 Deg paralogues are encoded by the genome of Arabidopsis thaliana, six of which were experimentally shown to be located in chloroplasts, one in peroxisomes, one in mitochondria and one in the nucleus. Two more Deg proteases are predicted to reside in chloroplasts, five in mitochondria (one of them with a dual chloroplastidial/mitochondrial localization) and the subcellular location of one protein is uncertain. This review summarizes the current knowledge on the role of Deg proteases in maintaining protein homeostasis and protein processing in various subcompartments of the plant cell. The chloroplast Deg proteases are the best examined so far, especially with respect to their role in the degradation of photodamaged photosynthetic proteins and in biogenesis of photosystem II (PSII). A combined action of thylakoid lumen and stroma Deg proteases in the primary cleavage of photodamaged D1 protein from PSII reaction centre is discussed on the basis of a recently resolved crystal structure of plant Deg1. The peroxisomal Deg protease is a processing enzyme responsible for the cleavage of N-terminal peroxisomal targeting signals (PTSs). A. thaliana mutants lacking this enzyme show reduced peroxisomal β-oxidation, indicating for the first time the impact of protein processing on peroxisomal functions in plants. Much less data is available for mitochondrial and nuclear Deg proteases. Based on the available expression data we hypothesize a role in general protein quality control and during acquired heat resistance.

Conditional Depletion of the Chlamydomonas Chloroplast ClpP Protease Activates Nuclear Genes Involved in Autophagy and Plastid Protein Quality Control

“…During assembly of the photosynthetic apparatus, most protein subunits that are produced in excess are also rapidly degraded. The plastid contains several proteases including stromal ClpP (Olinares et al, 2011), stromal and thylakoid Deg (Schuhmann and Adamska, 2012), thylakoid FtsH (Adam et al, 2001;Sokolenko et al, 2002;Peltier et al, 2004), the intramembrane rhomboid proteases (Adam, 2013), and the thylakoid-bound SppA (Lensch et al, 2001) and Egy1 proteases (Chen et al, 2005). Moreover, several processing proteases exist in these organelles, such as SPP (stromal processing peptidase) and TPP (thylakoid processing protease), which are required for cleaving the transit peptide and the thylakoid targeting domain, respectively (Oelmüller et al, 1996;Richter and Lamppa, 1998).…”

Downregulation of chloroplast protease AtDeg5 leads to changes in chronological progression of ontogenetic stages, leaf morphology and chloroplast ultrastructure in Arabidopsis

“…It has been experimentally proven that five of the genes (AtDEG1, 2, 5, 7 and 8) encode proteins targeted to chloroplasts, one (AtDEG10) encodes a mitochondriontargeted protein and another one (AtDEG15) a peroxisomal protein. The remaining nine AtDEG genes code for proteins which have been predicted in silico to be targeted either to mitochondria or chloroplasts or their localization has not been predicted in silico yet (for a review see [9]). …”

Organellar oligopeptidase (OOP) provides a complementary pathway for targeting peptide degradation in mitochondria and chloroplasts

“…In turn, chloroplastic and mitochondrial protein degradation is carried out by proteases of bacterial origin belonging to the serine (Lon, ClpP, Deg) or metalloprotease (FtsH) type families (reviewed in refs. [4][5][6]. To date ∼35 different proteases in the chloroplasts and mitochondria of A. thaliana have been identified as being involved in the removal of misfolded or oxidatively damaged proteins and in the degradation of unassembled subunits of protein complexes.…”