title: Comparative Analysis of the Flax Immune Receptors L6 and L7 Suggests an Equilibrium-Based Switch Activation Model.
abstract: NOD-like receptors (NLRs) are central components of the plant immune system. L6 is a Toll/interleukin-1 receptor (TIR) domain-containing NLR from flax (Linum usitatissimum) conferring immunity to the flax rust fungus. Comparison of L6 to the weaker allele L7 identified two polymorphic regions in the TIR and the nucleotide binding (NB) domains that regulate both effector ligand-dependent and -independent cell death signaling as well as nucleotide binding to the receptor. This suggests that a negative functional interaction between the TIR and NB domains holds L7 in an inactive/ADP-bound state more tightly than L6, hence decreasing its capacity to adopt the active/ATP-bound state and explaining its weaker activity in planta. L6 and L7 variants with a more stable ADP-bound state failed to bind to AvrL567 in yeast two-hybrid assays, while binding was detected to the signaling active variants. This contrasts with current models predicting that effectors bind to inactive receptors to trigger activation. Based on the correlation between nucleotide binding, effector interaction, and immune signaling properties of L6/L7 variants, we propose that NLRs exist in an equilibrium between ON and OFF states and that effector binding to the ON state stabilizes this conformation, thereby shifting the equilibrium toward the active form of the receptor to trigger defense signaling.
authors.affiliation: CSIRO Agriculture, Canberra ACT 2601, Australia maud.bernoux@csiro.au.
authors.forename: Maud
authors.identifier: http://orcid.org/0000-0001-5416-4295
authors.initials: M
authors.lastname: Bernoux
authors.affiliation: School of Biological Sciences, Flinders University, Adelaide SA 5001, Australia.
authors.forename: Hayden
authors.identifier: http://orcid.org/0000-0003-4803-9660
authors.initials: H
authors.lastname: Burdett
authors.affiliation: School of Chemistry and Molecular Biosciences, Institute for Molecular Bioscience and Australian Infectious Diseases Research Centre, University of Queensland, Brisbane QLD 4072, Australia.
authors.forename: Simon J
authors.identifier: http://orcid.org/0000-0003-4781-6261
authors.initials: SJ
authors.lastname: Williams
authors.affiliation: CSIRO Agriculture, Canberra ACT 2601, Australia.
authors.forename: Xiaoxiao
authors.identifier: http://orcid.org/0000-0002-5417-6963
authors.initials: X
authors.lastname: Zhang
authors.affiliation: CSIRO Agriculture, Canberra ACT 2601, Australia.
authors.forename: Chunhong
authors.initials: C
authors.lastname: Chen
authors.affiliation: CSIRO Agriculture, Canberra ACT 2601, Australia.
authors.forename: Kim
authors.initials: K
authors.lastname: Newell
authors.affiliation: CSIRO Agriculture, Canberra ACT 2601, Australia.
authors.forename: Gregory J
authors.initials: GJ
authors.lastname: Lawrence
authors.affiliation: School of Chemistry and Molecular Biosciences, Institute for Molecular Bioscience and Australian Infectious Diseases Research Centre, University of Queensland, Brisbane QLD 4072, Australia.
authors.forename: Bostjan
authors.identifier: http://orcid.org/0000-0001-9413-9166
authors.initials: B
authors.lastname: Kobe
authors.affiliation: CSIRO Agriculture, Canberra ACT 2601, Australia.
authors.forename: Jeffrey G
authors.initials: JG
authors.lastname: Ellis
authors.affiliation: School of Biological Sciences, Flinders University, Adelaide SA 5001, Australia.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Anderson
authors.affiliation: CSIRO Agriculture, Canberra ACT 2601, Australia.
authors.forename: Peter N
authors.initials: PN
authors.lastname: Dodds
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D016923:Cell Death; D019597:Flax / Q000166:cytology / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D010641:Phenotype; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism*; D011110:Polymorphism, Genetic; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011956:Receptors, Cell Surface / Q000737:chemistry / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D016415:Sequence Alignment
chemical_list: D010940:Plant Proteins; D011956:Receptors, Cell Surface; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate