HPE1, an effector from zebra chip pathogen interacts with tomato proteins and perturbs ubiquitinated protein accumulation

Authors: KAN, CHIA-CHENG - Texas A&M University; MENDOZA-HERRERA, AZUCENA - Texas A&M University; LEVY, JULIEN - Texas A&M University; Hull, Joe; Fabrick, Jeffrey; TAMBORINDEGUY, CECILIA - Texas A&M University

Submitted to: International Journal of Molecular Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/16/2021
Publication Date: 8/20/2021
Citation: Kan, C., Mendoza-Herrera, A., Levy, J., Hull, J.J., Fabrick, J.A., Tamborindeguy, C. 2021. HPE1, an effector from zebra chip pathogen interacts with tomato proteins and perturbs ubiquitinated protein accumulation. International Journal of Molecular Sciences. 22(16). Article 9003. https://doi.org/10.3390/ijms22169003.
DOI: https://doi.org/10.3390/ijms22169003

Interpretive Summary: Liberibacter are phloem-limited bacteria transmitted by psyllids that are pathogenic for plants and are the underlying cause of citrus greening disease (also known as Huanglongbing) in citrus and zebra chip disease in potatoes. A number of studies have shown that many plant pathogens secrete proteins to not only overcome the host plant immune response but to also manipulate host cellular functions to their own benefit. A Liberibacter protein termed HPE1 was previously shown to suppress portions of the plant’s immune response. In this study, HPE1 overexpression was found to induce protein ubiquitination, a cellular process that leads to protein degradation. Similar ubiquitination was observed in Liberibacter infected tomato plants and HPE1 overexpressing plants exhibited phenotypic symptoms that were comparable to those observed following Liberibacter infection. Using a yeast two-hybrid system, tomato plant RAD23 proteins were identified as HPE1 interactors. Overall, the study suggests a virulent factor role for HPE1 in which the Liberibacter protein perturbs the plant’s ubiquitin-proteasome system through direct interactions with RAD23 proteins.

Technical Abstract: The gram-negative bacterial genus Liberibacter includes economically important pathogens, such as ‘Candidatus Liberibacter asiaticus’ that cause citrus greening disease (or Huanglongbing, HLB) and ‘Ca. Liberibacter solanacearum’ (Lso) that cause zebra chip disease in potato. Liberibacter pathogens are fastidious bacteria transmitted by psyllids. Pathogen manipulation of the host’ and vector’s immune system for successful colonization is hypothesized to be achieved by Sec translocon-dependent effectors (SDE). In previous work, we identified hypothetical protein effector 1 (HPE1), an SDE from Lso, that acts as a suppressor of the plant’s effector-triggered immunity (ETI)-like response. In this study, using a yeast two-hybrid system, we identify binding interactions between tomato RAD23 proteins and HPE1. We further show that HPE1 interacts with RAD23 in both nuclear and cytoplasmic compartments in planta. Immunoblot assays show that HPE1 is not ubiquitinated in the plant cell, but rather the expression of HPE1 induced the accumulation of other ubiquitinated proteins. A similar accumulation of ubiquitinated proteins is also observed in Lso infected tomato plants. Finally, earlier colonization and symptom development following Lso haplotype B infection are observed in HPE1 overexpressing plants compared to wild-type plants. Overall, our results suggest that HPE1 plays a role in virulence in Lso pathogenesis, possibly by perturbing the ubiquitin-proteasome system via direct interaction with the ubiquitin-like domain of RAD23 proteins.