BTB-BACK Domain Protein POB1 Suppresses Immune Cell Death by Targeting Ubiquitin E3 ligase PUB17 for Degradation.

Beatriz Orosa,Hazel Mclellan,Chengwei Yang,Mark Bailey,Eleanor M Gilroy,Qin He,Cunjin Zhang,Joelle Mesmar,Zhendong Tian,Adam Craig,Paul R J Birch,Paul R J Birch,Jonathan David Moore,Ari Sadanandom,Petra C Boevink

doi:10.1371/journal.pgen.1006540

Abstract

Hypersensitive response programmed cell death (HR-PCD) is a critical feature in plant immunity required for pathogen restriction and prevention of disease development. The precise control of this process is paramount to cell survival and an effective immune response. The discovery of new components that function to suppress HR-PCD will be instrumental in understanding the regulation of this fundamental mechanism. Here we report the identification and characterisation of a BTB domain E3 ligase protein, POB1, that functions to suppress HR-PCD triggered by evolutionarily diverse pathogens. Nicotiana benthamiana and tobacco plants with reduced POB1 activity show accelerated HR-PCD whilst those with increased POB1 levels show attenuated HR-PCD. We demonstrate that POB1 dimerization and nuclear localization are vital for its function in HR-PCD suppression. Using protein-protein interaction assays, we identify the Plant U-Box E3 ligase PUB17, a well established positive regulator of plant innate immunity, as a target for POB1-mediated proteasomal degradation. Using confocal imaging and in planta immunoprecipitation assays we show that POB1 interacts with PUB17 in the nucleus and stimulates its degradation. Mutated versions of POB1 that show reduced interaction with PUB17 fail to suppress HR-PCD, indicating that POB1-mediated degradation of PUB17 U-box E3 ligase is an important step for negative regulation of specific immune pathways in plants. Our data reveals a new mechanism for BTB domain proteins in suppressing HR-PCD in plant innate immune responses.

Highlights

The capacity of plants to protect themselves against pathogens depends on detection mechanisms that recognize pathogen-derived molecules and activate host defence responses
There is a real paucity in understanding of the biochemical processes that are crucial in determining how programmed cell death (PCD) is co-ordinated during plant immune
We demonstrate that POB1 dimerization and nuclear localization are required for its function in Hypersensitive response programmed cell death (HR-PCD) suppression

Summary

Introduction

The capacity of plants to protect themselves against pathogens depends on detection mechanisms that recognize pathogen-derived molecules and activate host defence responses. Plants have evolved an armoury of defence mechanisms that allow them to counter infection. These encompass both basal responses, triggered by recognition of conserved pathogen-associated molecular patterns (PAMPs), and pathogen-specific responses, mediated via pathogenand plant-specific gene-for-gene recognition events. Recognition leads to activation of immune responses [1]. One feature of the immune response in plants is the hypersensitive response (HR). This involves a highly localized programmed cell death (PCD) of the infected region that helps to contain pathogen spread [2]. Understanding how infected cells bring about downstream molecular signalling to establish and regulate cell death is a fundamental challenge in plant biology

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