Regulation of immune receptor homeostasis : lessons from four MUSE proteins

Plants are sessile organisms and rely on their innate immune systems to defend against pathogens. Nucleotide-binding (NB) and leucine-rich repeat (LRR) domain-containing proteins (NLRs) serve as immune receptors in both plants and animals. It remains poorly understood how NLRs are activated and very few components downstream of NLR activation have been identified. Autoimmune mutants provide a powerful tool to study NLR-mediated immune pathways through genetic screens. To search for negative regulators of plant immunity, we performed a MUSE (MUTANT, SNC1-ENHANCING) screen in Arabidopsis thaliana, where 15 novel muse mutants were identified. My Ph.D. dissertation consists of the cloning and characterization of four muse mutants: muse10, muse12, muse13 and muse14. muse10 and muse12 are alleles of Arabidopsis thaliana hsp90.3 and hsp90.2, respectively. HSP90s are conserved molecular chaperones that play diverse roles. Previous studies have shown that HSP90s are positive regulators in NLR protein assembly. We found that they also exert negative roles in NLR protein turnover. Specific mutations in HSP90.3 lead to increased protein levels of multiples NLRs. Using an immunoprecipitation assay we also demonstrated that SNC1 is a client of HSP90.3. MUSE13 and MUSE14 are both TRAF domain-containing proteins. TRAF domains are conserved in eukaryotes, and TRAF domain proteins are predominantly involved in protein ubiquitination or protein processing. MUSE13 and MUSE14 function redundantly in the negative regulation of protein turnover of two NLRs. We found that MUSE13 forms a protein complex with SCFCPR1 and NLRs, which suggests the existence of a plant type TRAFasome that modulates NLR homeostasis. Overall, my study on these MUSE proteins provides insights into the regulation of plant NLR turnover, and broadens our knowledge of the tight control of the plant immune system.Science, Faculty ofBotany, Department ofGraduat