A bacterial E3 ubiquitin ligase targets a host protein kinase to disrupt plant immunity

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Abstract
Pseudomonas syringae, the cause of bacterial speck disease in tomatoes, injects an array of effector molecules into the host cell in order to secure its infection. One of them, AvrPtoB, has now been found to target the host kinase Fen, part of a highly conserved plant immunity pathway. To do this, AvrPtoB employs E3 ubiquitin ligase activity, which promotes protein degradation, and must have been acquired by this bacterial pathogen at some stage in evolution for this purpose. Pseudomonas syringae translocates many effectors into the plant host cell on infection, one of which is AvrPtoB. This E3 ubiquitin ligase has now been demonstrated to specifically modulate the host kinase Fen, which is involved in plant resistance. Many bacterial pathogens of plants and animals use a type III secretion system to deliver diverse virulence-associated ‘effector’ proteins into the host cell1. The mechanisms by which these effectors act are mostly unknown; however, they often promote disease by suppressing host immunity2. One type III effector, AvrPtoB, expressed by the plant pathogen Pseudomonas syringae pv. tomato, has a carboxy-terminal domain that is an E3 ubiquitin ligase3. Deletion of this domain allows an amino-terminal region of AvrPtoB (AvrPtoB1–387) to be detected by certain tomato varieties leading to immunity-associated programmed cell death4. Here we show that a host kinase, Fen, physically interacts with AvrPtoB1–387 and is responsible for activating the plant immune response. The AvrPtoB E3 ligase specifically ubiquitinates Fen and promotes its degradation in a proteasome-dependent manner. This degradation leads to disease susceptibility in Fen-expressing tomato lines. Various wild species of tomato were found to exhibit immunity in response to AvrPtoB1–387 and not to full-length AvrPtoB. Thus, by acquiring an E3 ligase domain, AvrPtoB has thwarted a highly conserved host resistance mechanism.