Proteasome-associated HECT-type ubiquitin ligase activity is required for plant immunity.

James J Furniss,Yasuomi Tada,Zhishuo Wang,Steven H Spoel,Mika Nomoto,Heather Grey,Lorna Jackson,Libo Shan

doi:10.1371/journal.ppat.1007447

James J Furniss, Yasuomi Tada + Show 6 more

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https://doi.org/10.1371/journal.ppat.1007447

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Journal: PLoS Pathogens	Publication Date: Nov 20, 2018
Citations: 48	License type: CC BY 4.0

Affiliation: University of Edinburgh, Nagoya University

Abstract

Regulated degradation of proteins by the 26S proteasome plays important roles in maintenance and signalling in eukaryotic cells. Proteins are marked for degradation by the action of E3 ligases that site-specifically modify their substrates by adding chains of ubiquitin. Innate immune signalling in plants is deeply reliant on the ubiquitin-26S proteasome system. While progress has been made in understanding substrate ubiquitination during plant immunity, how these substrates are processed upon arrival at the proteasome remains unclear. Here we show that specific members of the HECT domain-containing family of ubiquitin protein ligases (UPL) play important roles in proteasomal substrate processing during plant immunity. Mutations in UPL1, UPL3 and UPL5 significantly diminished immune responses activated by the immune hormone salicylic acid (SA). In depth analyses of upl3 mutants indicated that these plants were impaired in reprogramming of nearly the entire SA-induced transcriptome and failed to establish immunity against a hemi-biotrophic pathogen. UPL3 was found to physically interact with the regulatory particle of the proteasome and with other ubiquitin-26S proteasome pathway components. In agreement, we demonstrate that UPL3 enabled proteasomes to form polyubiquitin chains, thereby regulating total cellular polyubiquitination levels. Taken together, our findings suggest that proteasome-associated ubiquitin ligase activity of UPL3 promotes proteasomal processivity and is indispensable for development of plant immunity.

Highlights

The ubiquitin-26S proteasome system (UPS) plays an essential cellular role in selective degradation of proteins that are short-lived or damaged
In particular we show that UPL3 associated with proteasomal degradation pathway components and provided the proteasome with ubiquitin ligase activity, which was necessary for large scale salicylic acid (SA)-induced transcriptional reprogramming and immunity
The Arabidopsis ubiquitin protein ligases (UPL) family consists of 7 members that all contain a C-terminal HECT domain that accepts ubiquitin from an E2 conjugating enzyme and transfers it to the target substrate

Summary

Introduction

The ubiquitin-26S proteasome system (UPS) plays an essential cellular role in selective degradation of proteins that are short-lived or damaged. Degradation of proteins is mediated by an enzymatic cascade in which a small and highly conserved ubiquitin molecule is covalently attached to the substrate. In partnership with an E3 ligase that recruits a specific substrate, the E2 enzyme facilitates formation of an isopeptide bond between the ε-amino group of a lysine residue within the substrate and the carboxy-terminal group of ubiquitin. Reiterations of this reaction cycle result in subsequent ubiquitin molecules being attached to internal lysines of the preceding ubiquitin moiety, thereby generating a polyubiquitin chain on the substrate [1, 2]. Substrate degradation involves its unfolding by chaperone activity of the 19S particle, cleavage and release of the polyubiquitin chain for recycling, and subsequent threading of the unfolded substrate into the 20S subunit of the proteasome, a barrel-shaped multi-catalytic proteinase [4]

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