Structure and Function of a Guarded Arabidopsis Immune Signaling Ubiquitin Ligase

Abstract

The Arabidopsis thaliana E3 ubiquitin ligase SAUL1 (SENESCENCE-ASSOCIATED E3 UBIQUITIN LIGASE 1), which acts as a positive regulator during pattern-triggered immunity (PTI), is characterized by two intriguing hallmarks. (i) On the one hand, SAUL1 is guarded by two heteromeric nucleotide-binding leucine-rich repeat protein (NLR) complexes. These initiate an inducible effector-triggered immunity (ETI) in the saul1 1 mutant. Although ETI is known now for more than 15 years its early regulations and the demarcation from PTI are still not fully understood. Therefore, the saul1 1 phenotype was used to investigate gene regulations during ETI by transcriptomics. These analyses revealed that observed gene regulations were highly similar to other autoimmune mutants and saul1 1 is therefore an ideal model to study ETI. By accessing global changes an early interplay of multiple plant hormones, like salicylic acid, jasmonic acid, ethylene and abscisic acid, was identified, which is likely to initiate the onset of ETI. In addition, repression of brassinosteroid (BR) signaling may be a potential reason of the autoimmune-related growth arrest in saul1 1. Before, BR signaling, as well as synthesis of camalexin and JA signaling have been associated exclusively with PTI. In conclusion, it was possible to show that ETI and PTI are not as distinct as originally thought. In addition, 19 very early differentially expressed genes were identified during the first two hours of the onset of the saul1 1 phenotype. Five of them were analyzed concerning their potential driving role during ETI and two transcription factors, ERF2 (Ethylene Response Factor 2) and ZAT7 (Zinc Finger of Arabidopsis thaliana 7), were identified that pose ideal candidates being initial regulators of ETI. (ii) On the other hand, SAUL1 is characterized by its plasma membrane localization and its domain structure which differ from other plant U-box type E3 ligases (PUBs). Structural experiments allowed to determine for the first time the structure of a PUB. This revealed that SAUL1 is most likely present as a U box-mediated con¬centration-dependent oligomer, which is mostly present as a dimer and tetramer. Therefore, SAUL1 activity may be controlled by oligomerization rendering the U box inaccessible in a multimeric state. Analyzing the domain organization, it was possible to identify a positively charged stretch next to the C terminus that is likely to bind to negatively charged phospholipids. Therefore, and due to the predicted flexibility of the C-terminal armadillo repeats, SAUL1 may bind to multi-vesicular bodies (MVBs) and to the plasma membrane at the same time. This could result in the observed patches at the plasma membrane, originating from MVB tethering, which would be regulated by oligomerization as well. In addition, the low-resolution structure of the SAUL1 binding partner BON1, a plant copine, was as well determined. Although no direct interaction between SAUL1 and BON1 could be observed, the performed experiments may suggest that Ca2+ ions may mediate a SAUL1-BON1 interaction. In conclusion, this thesis provides new insights into regulations during ETI, the structural organization of SAUL1, the concentration-dependent oligomerization of SAUL1, its membrane binding capacities and the interaction between SAUL1 and BON1.