The HopZ Family ofPseudomonas syringaeType III Effectors Require Myristoylation for Virulence and Avirulence Functions inArabidopsis thaliana

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Abstract
Pseudomonas syringaeutilizes the type III secretion system to translocate effector proteins into plant cells, where they can contribute to the pathogen's ability to infect and cause disease. Recognition of these effectors by resistance proteins induces defense responses that typically include a programmed cell death reaction called the hypersensitive response. The YopJ/HopZ family of type III effector proteins is a common family of effector proteins found in animal- and plant-pathogenic bacteria. The HopZ family inP. syringaeincludes HopZ1aPsyA2, HopZ1bPgyUnB647, HopZ1cPmaE54326, HopZ2Ppi895Aand HopZ3PsyB728a. HopZ1a is predicted to be most similar to the ancestralhopZallele and causes a hypersensitive response in multiple plant species, includingArabidopsis thaliana. Therefore, it has been proposed that host defense responses have driven the diversification of this effector family. In this study, we further characterized the hypersensitive response induced by HopZ1a and demonstrated that it is not dependent on known resistance genes. Further, we identified a novel virulence function for HopZ2 that requires the catalytic cysteine demonstrated to be required for protease activity. Sequence analysis of the HopZ family revealed the presence of a predicted myristoylation sequence in all members except HopZ3. We demonstrated that the myristoylation site is required for membrane localization of this effector family and contributes to the virulence and avirulence activities of HopZ2 and HopZ1a, respectively. This paper provides insight into the selective pressures driving virulence protein evolution by describing a detailed functional characterization of the diverse HopZ family of type III effectors with the model plantArabidopsis.