Increases in activity of proteasome and papain-like cysteine protease in Arabidopsis autophagy mutants: back-up compensatory effect or cell-death promoting effect?

Marien Havé,Jean-Christophe Avice,Loïc Rajjou,Gwendal Cueff,Betty Cottyn-Boitte,Nico Dissmeyer,Aurélia Lornac,Patrick Gallois,Michel Zivy,Céline Masclaux-Daubresse,Pavel Reichman,Fabienne Soulay,Thierry Balliau,Emeline Dérond

doi:10.1093/jxb/erx482

Abstract

Autophagy is essential for protein degradation, nutrient recycling, and nitrogen remobilization. Autophagy is induced during leaf ageing and in response to nitrogen starvation, and is known to play a fundamental role in nutrient recycling for remobilization and seed filling. Accordingly, ageing leaves of Arabidopsis autophagy mutants (atg) have been shown to over-accumulate proteins and peptides, possibly because of a reduced protein degradation capacity. Surprisingly, atg leaves also displayed higher protease activities. The work reported here aimed at identifying the nature of the proteases and protease activities that accumulated differentially (higher or lower) in the atg mutants. Protease identification was performed using shotgun LC-MS/MS proteome analyses and activity-based protein profiling (ABPP). The results showed that the chloroplast FTSH (FILAMENTATION TEMPERATURE SENSITIVE H) and DEG (DEGRADATION OF PERIPLASMIC PROTEINS) proteases and several extracellular serine proteases [subtilases (SBTs) and serine carboxypeptidase-like (SCPL) proteases] were less abundant in atg5 mutants. By contrast, proteasome-related proteins and cytosolic or vacuole cysteine proteases were more abundant in atg5 mutants. Rubisco degradation assays and ABPP showed that the activities of proteasome and papain-like cysteine protease were increased in atg5 mutants. Whether these proteases play a back-up role in nutrient recycling and remobilization in atg mutants or act to promote cell death is discussed in relation to their accumulation patterns in the atg5 mutant compared with the salicylic acid-depleted atg5/sid2 double-mutant, and in low nitrate compared with high nitrate conditions. Several of the proteins identified are indeed known as senescence- and stress-related proteases or as spontaneous cell-death triggering factors.

Highlights

Autophagy is a universal mechanism that facilitates the degradation of unwanted cell constituents in the lytic compartments of eukaryotic cells
Whether these proteases play a back-up role in nutrient recycling and remobilization in atg mutants or act to promote cell death is discussed in relation to their accumulation patterns in the atg5 mutant compared with the salicylic acid-depleted atg5/sid2 double-mutant, and in low nitrate compared with high nitrate conditions
The aim of this study was to identify protease activities up-regulated in autophagy mutants in order to further determine their roles in nitrogen remobilization for seed filling (Guiboileau et al, 2012)

Summary

Introduction

Autophagy is a universal mechanism that facilitates the degradation of unwanted cell constituents in the lytic compartments of eukaryotic cells. Among the 50 ATG genes discovered in yeast, 18 are part of the autophagy core machinery (Yang and Bassham, 2015) These genes are absolutely essential to the formation of autophagosomes. Sucrose, and hexose contents in their leaves and higher amounts of amino acids (especially glutamate, aspartate, and methionine) They display higher protein and RNA concentrations relative to wild-type plants. The results showed that several proteins and peptides were accumulated in the leaves of atg mutants. Protease activities were significantly increased in the leaves of atg mutants (Guiboileau et al, 2013), showing that the accumulation of the peptides and proteins over-abundant in the atg rosettes was not related to lower proteolysis capacity. In was hypothesized that in the absence of autophagosome trafficking, vacuole proteases and their cytoplasmic substrates cannot co-localize, resulting in protein over-accumulation

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