A unique TIR- receptor is required for recognition of a bacterial type-III effector 
R. ANDERSON (1), M. Nishimura (2), K. Cherkis (3), M. Machius (3), Q. Lui (4), T. Law (2), J. Dangl (1) (1) University of North Carolina at Chapel Hill, U.S.A.; (2) University of North Carolina at Chapel Hill, U.S.A.; (3) University of North Carolina at Chapel Hill, U.S.A.; (4) University of North Carolina at Chapel Hill, U.S.A.; (5) University of North Carolina at Chapel Hill, U.S.A.

The ability to monitor signals of microbial invasion is universally the first step in mounting an immune response. In plants, the intracellular “sensor” proteins of the nucleotide-binding domain (NBD) leucine-rich repeat (LRR) family (NLRs) are critical components for activating an axis of plant immunity known as effector-triggered immunity (ETI). Using a genetic approach of exploiting natural variation, we isolated Response to HopBA1 (RBA1), a novel single TOLL/interleukin-1 receptor (TIR) domain protein. RBA1 is required for the recognition of HopBA1, a Pseudomonas type III effector (TTE). We solved the crystal structure of HopBA1 and found striking resemblance to a bacterial heme scavenger protein.  Mutations in and around the putative heme binding site of HopBA1 result in loss of recognition and physical association with RBA1. Akin to the multidomain TIR NLRs (TNLs), Arabidopsis RPS4 and flax L6, RBA1 self-association is required for function. Remarkably, mutations in the distinct putative RPS4 or L6 dimer interfaces abolish RBA1 function.  Furthermore, RBA1-HopBA1 interaction promotes RBA1 self-association, while mutations in the predicted RBA1 dimer interfaces abolish the HopBA1 interaction. The absence of the additional auto-regulatory domains found on canonical NLR receptors prompted us to explore the molecular mechanisms of RBA1 regulation. Our findings demonstrate an immune function for previously uncharacterized "truncated" NLRs and encourage us to reconsider our current understanding of NLR receptor domain structure and oligomerization. 

Abstract Number: P17-487
Session Type: Poster