Differential N-end rule degradation of RIN4/NOI fragments generated by the AvrRpt2 effector protease

Abstract

One sentence summary: Analysis of RIN4/NOI fragments released after cleavage by the bacterial effector protease AvrRpt2 reveals a novel role of the N-end rule in the degradation of NOI-domain proteins, but not of RIN4.Author contributions: KG, AK and EG constructed plasmids used in tobacco and Arabidopsis protoplast experiments, including tFT plasmids; KG carried out tobacco transient expression experiments, immunoblots and confocal imaging, as well as all immunoblots for RIN4; LE-L conducted all NOI experiments in Arabidopsis protoplasts, including immunoblots; CN and MK constructed plasmids for in vitro arginylation assays and performed these assays; EL conducted tFT experiments; MS and RdM carried out inoculations with Pst AvrRpt2 and confocal analysis; EG, ND, JL and MW supervised the experiments; EG wrote the article with contributions of all the authors; EG agrees to serve as the author responsible for contact and ensures communication. AbstractThe protein RPM1-INTERACTING PROTEIN4 (RIN4) is a central regulator of both layers of plant immunity systems, the so-called pattern-triggered immunity (PTI) and effectortriggered immunity (ETI). RIN4 is targeted by several effectors, including the Pseudomonas syringae protease effector AvrRpt2. Cleavage of RIN4 by AvrRpt2 generates unstable RIN4 fragments, whose degradation leads to the activation of the resistance protein RPS2 (RESISTANT TO P. SYRINGAE2). Hence, identifying the determinants of RIN4 degradation is key to understanding RPS2-mediated ETI, as well as virulence functions of AvrRpt2. In addition to RIN4, AvrRpt2 cleaves host proteins from the nitrate-induced (NOI) domain family. Although cleavage of NOI-domain proteins by AvrRpt2 may contribute to PTI regulation, the (in)stability of these proteolytic fragments and the determinants that regulate their stability have not been examined. Notably, a common feature of RIN4 and of many NOI-domain protein fragments generated by AvrRpt2 cleavage is the exposure of a new N-terminal residue that is destabilizing according to the N-end rule. Using antibodies raised against endogenous RIN4, we show that the destabilization of AvrRpt2-cleaved RIN4 fragments is independent of the N-end rule pathway (recently renamed N-degron pathway). By contrast, several NOI-domain protein fragments are bona fide substrates of the N-degron pathway. The discovery of this novel set of substrates considerably expands the number of proteins targeted for degradation by this ubiquitin-dependent pathway, for which very few physiological substrates are known in plants. Our results also open new avenues of research to understand the role of AvrRpt2 in promoting bacterial virulence.