AvrBsT Acetylates Arabidopsis ACIP1, a Protein that Associates with Microtubules and Is Required for Immunity

Bacterial pathogens of plant and animals share a homologous group of virulence factors, referred to as the YopJ effector family, which are translocated by the type III secretion (T3S) system into host cells during infection. Recent work indicates that some of these effectors encode acetyltransferases that suppress host immunity. The YopJ-like protein AvrBsT is known to activate effector-triggered immunity (ETI) in Arabidopsis thaliana Pi-0 plants; however, the nature of its enzymatic activity and host target(s) has remained elusive. Here we report that AvrBsT possesses acetyltransferase activity and acetylates ACIP1 (for ACETYLATED INTERACTING PROTEIN1), an unknown protein from Arabidopsis. Genetic studies revealed that Arabidopsis ACIP family members are required for both pathogen-associated molecular pattern (PAMP)-triggered immunity and AvrBsT-triggered ETI during Pseudomonas syringae pathovar tomato DC3000 (Pst DC3000) infection. Microscopy studies revealed that ACIP1 is associated with punctae on the cell cortex and some of these punctae co-localize with microtubules. These structures were dramatically altered during infection. Pst DC3000 or Pst DC3000 AvrRpt2 infection triggered the formation of numerous, small ACIP1 punctae and rods. By contrast, Pst DC3000 AvrBsT infection primarily triggered the formation of large GFP-ACIP1 aggregates, in an acetyltransferase-dependent manner. Our data reveal that members of the ACIP family are new components of the defense machinery required for anti-bacterial immunity. They also suggest that AvrBsT-dependent acetylation in planta alters ACIP1's defense function, which is linked to the activation of ETI. Author Summary How host disease resistance pathways are activated in response to pathogens remains a fundamental question in host-pathogen interactions. In this work, we used the Pseudomonas-Arabidopsis pathosystem to study how the AvrBsT effector activates plant immune signaling. AvrBsT belongs to the YopJ effector family, a group of virulence proteins shared by bacterial pathogens of plants and animals. Bacteria inject these effectors into plant or animal host cells to promote pathogenesis. Recent biochemical studies show that several members of the YopJ family encode acetyltransferases that acetylate host proteins to suppress immune signaling. How the immune system specifically recognizes this family of effectors and/or monitors host acetylation is poorly understood. In this work, we provide biochemical evidence that AvrBsT is an acetyltransferase. We also report the identification and characterization of ACIP1, an Arabidopsis protein of unknown function that is an AvrBsT substrate. We provide evidence that ACIP1 is required for plant immunity and its association with microtubules changes during infection. Moreover, our work suggests that AvrBsT acetyltransferase in planta leads to dramatic changes in ACIP1 localization, which coincides with the activation of strong defense responses. This study highlights an important link between ACIP1 and the microtubule network during anti-bacterial immunity.