Effector-mediated acetylation

Effector acetylation and activation of plant immunity:
In the past few years, effector-mediated acetylation of host proteins has emerged as a key virulence strategy to alter immune signaling. Pioneering biochemical work from Kim Orth’s laboratory revealed that YopJ from Yersinia encodes a potent acetyltransferase that inhibits MAPK activation in mammalian cells (Mukherjee et al., 2006). Subsequent studies from my group and others have confirmed that YopJ-like effectors conserved and diversified in phytopathogenic bacteria also utilize acetyltransferase activity to manipulate their hosts (Cheong et al., 2014; Jiang et al., 2013; Lee et al., 2012; Tasset et al., 2010), further highlighting the importance of acetylation in bacterial-plant interactions. What is striking is the diversity of plant substrates found to be acetylated by YopJ-like effectors (Cheong et al., 2014; Jiang et al., 2013; Lee et al., 2012). These findings suggest that phytopathogenic bacteria may have exploited post-translational acetylation as a mechanism to interfere with multiple, key nodes in plant immune signaling.

How the immune system recognizes YopJ or YopJ-like effectors and/or monitors host acetylation is poorly understood. To gain insight here, we exploited the use of the Pseudomonas-Arabidopsis pathosystem to study how AvrBsT, a YopJ-like effector from Xe, activates immune signaling (Cunnac et al., 2007; Kirik and Mudgett, 2009). Defining AvrBsT-elicited responses in resistant Arabidopsis lines (Cunnac et al., 2007) enabled us to genetically and biochemically study AvrBsT-triggered ETI and isolate potential AvrBsT substrates. In recent work, we demonstrated that AvrBsT acetylates ACIP1 (for ACETYLATED INTERACTING PROTEIN1), a small a-helical protein from Arabidopsis (Cheong et al., 2014). We provided evidence that ACIP1 is a new component of the core defense machinery required for anti-bacterial immunity. We showed that large ACIP1-containing punctae are associated with the cortical microtubule network and their localization is changed in response to pathogen infection during PTI (Cheong et al., 2014). Moreover, we provided evidence that AvrBsT-dependent perturbation of ACIP1 localization correlates with AvrBsT-triggered immune responses (Cheong et al., 2014). Our work thus highlights an important new link between ACIP1 and the microtubule network during plant immune signaling. It also reveals that perturbation of this node during infection is highly monitored by the plant immune system.

Ongoing work will further characterize ACIP1 cell biology to gain more insight to how acetylation of ACIP1 leads to ETI. In addition, we will explore the potential link between phosphatidic acid (PA), ACIP1, and the microtubule network during pathogen infection. Our work suggests that AvrBsT-triggered PA production (Kirik and Mudgett, 2009) might regulate ACIP1 complex formation and/or association with microtubules (Cheong et al., 2014). PA is known to a play a critical role in the regulation of cytoskeleton dynamics (Pleskot et al., 2013). How PA directly alters the microtubule network during ETI remains to be determined.