The truncated NLR protein TIR-NBS13 is a MOS6/IMPORTIN-α3 interaction partner required for plant immunity

Importin-alpha proteins mediate the translocation of nuclear localization signal (NLS)-containing proteins from the cytoplasm into the nucleus through nuclear pore complexes (NPCs). Genetically, Arabidopsis IMPORTIN-3/MOS6 (MODIFIER OF SNC1, 6) is required for basal plant immunity and constitutive disease resistance activated in autoimmune mutant snc1 (suppressor of npr1-1, constitutive 1), suggesting that MOS6 plays a role in the nuclear import of proteins involved in plant defense signaling. Here, we sought to identify and characterize defense-regulatory cargo proteins and interaction partners of MOS6. We conducted both in silico database analyses and affinity purification of functional epitope-tagged MOS6 from pathogen-challenged stable transgenic plants coupled with mass spectrometry. We show that among the 13 candidate MOS6 interactors we selected for further functional characterization, the TIR-NBS-type protein TN13 is required for resistance against Pseudomonas syringae pv. tomato (Pst) DC3000 lacking the type-III effector proteins AvrPto and AvrPtoB. When expressed transiently in N. benthamiana leaves, TN13 co-immunoprecipitates with MOS6, but not with its closest homolog IMPORTIN-alpha 6, and localizes to the endoplasmic reticulum (ER), consistent with a predicted N-terminal transmembrane domain in TN13. Our work uncovered the truncated NLR protein TN13 as a component of plant innate immunity that selectively binds to MOS6/IMPORTIN-alpha 3 in planta. We speculate that the release of TN13 from the ER membrane in response to pathogen stimulus, and its subsequent nuclear translocation, is important for plant defense signal transduction. Significance Statement Genetically, Arabidopsis IMPORTIN-3/MOS6 is required for basal plant immunity and constitutive resistance activated in the autoimmune mutant snc1, suggesting that MOS6 mediates the nuclear import of unknown proteins involved in plant defense signaling. In this study, we identified the truncated NLR protein TIR-NBS13 as an in planta interaction partner of MOS6 and a component of plant innate immunity.