SUSA2 Is an F-Box Protein Required for Autoimmunity Mediated by Paired NLRs SOC3-CHS1 and SOC3-TN2
This week we profile a recent publication in Nature Communications from Wanwan Liang (pictured, fifth from left)
in the laboratory of Dr. Xin Li (second row, third from right) at the Michael Smith Labs and UBC Botany.
Can you provide a brief overview of your lab’s current research focus?
The goal of our research program is to understand the molecular mechanisms of plant innate immunity. We study plant defense against pathogen infection in the context of gene regulation, protein-protein interaction and signal transduction using the model organism Arabidopsis thaliana. Our program aims to discover new regulatory components of plant disease resistance and to elucidate the biochemical functions of a number of regulators we have already identified. Understanding the innate ability of plants to defend themselves against pathogen infection promises to revolutionize disease control practices in our fields using environmentally friendly strategies.
What is the significance of the findings in this publication?
Both higher plants and mammals rely on nucleotide-binding leucine-rich repeat (NLR) immune receptors to detect pathogens and initiate immunity. Upon effector recognition, plant NLRs oligomerize for defense activation, the mechanism of which is poorly understood. We previously showed that disruption of the E3 ligase, Senescence-Associated E3 Ubiquitin Ligase 1 (SAUL1) leads to the activation of the NLR SOC3. Here, we report the identification of suppressor of saul1 2 (susa2) and susa3 from the saul1-1 suppressor screen. Pairwise interaction analysis suggests that both SUSA proteins interact with components of an SCFSUSA2 E3 ligase complex as well as CHS1 or TN2, truncated NLRs that pair with SOC3. susa2-2 only suppresses the autoimmunity mediated by either CHS1 or TN2, suggesting its specific involvement in SOC3-mediated immunity. In summary, our study indicates links between plant NLRs and an SCF complex that may enable ubiquitination and degradation of unknown downstream components to activate defense.
What are the next steps for this research?
1）Plants may use different NLR-SCF complexes to target certain conserved key negative regulators of immunity for degradation, triggering a common defense output. Future discovery of more F-box proteins involved in other NLR activation would enhance such prediction.
2）One key missing player in this model is the unknown ubiquitination substrate of SCFSUSA2. We tried to search for such substrate(s) using IP coupled to mass spectrometry (IP-MS) proteomics with DN-SUSA2 as bait. However, due to the very low abundance of the bait protein, our IP-MS did not yield any significant candidates. Future attempts with modern approaches feasible for low-abundance, transient or weak protein-protein interactions such as TurboID-based proximity labeling may help us identify the missing substrate of SUSA2.
This work was funded by:
Wanwan Liang is partly supported by a scholarship from China Scholarship Council (CSC). Research described is funded to Professor Xin Li by grants from NSERC-Discovery and CREATE (PRoTECT) programs, and CFI-JELF.