MOS6 and TN13 in plant immunity

Abstract

The Arabidopsis nuclear transport receptor IMPORTIN-α3/MOS6 (MODIFIER OF SNC1, 6) is required for constitutive defense responses of the auto-immune mutant snc1 (suppressor of npr1-1, constitutive 1) and contributes to basal disease resistance, suggesting a role in nuclear import of defense-regulatory cargo proteins. We recently showed that MOS6 selectively interacts with TN13, a TIR-NBS protein involved in basal resistance to Pseudomonas syringae pv. tomato (Pst) DC3000 lacking the effectors AvrPto and AvrPtoB. Consistent with a predicted N-terminal transmembrane domain, TN13 localizes to the endoplasmic reticulum (ER) and the nuclear envelope (NE) where it interacts with MOS6 in a transient expression assay. Here, we propose a model that summarizes the subcellular localization, association and function of TN13 and MOS6 in plant defense signaling.

Keywords: Arabidopsis; IMPORTIN-α3; MOS6; TIR-NBS13; plant immunity.

Figure 1.

MOS6 and TN13 in plant immunity. (A) Schematic representation of the predicted protein domain structure of TN13. Domains were predicted as previously described. NBS, nucleotide binding site; NLS, bipartite nuclear localization signal; TIR, Toll/Interleukin-1 Receptor homology domain; TM, transmembrane domain. (B) Hypothetical model summarizing the function, subcellular localization and association of TN13 and MOS6 in plant immunity. TN13 localizes to the ER membrane and the nuclear envelope through its N-terminal TM domain, where it associates with MOS6 in a preformed complex at the cytoplasmic side of the ER via its C-terminal bipartite NLS. Upon pathogen stimulus the TN13-MOS6 complex is released from the ER membrane allowing access of MOS6 to the second NLS at the N-terminus of TN13 to promote nuclear import together with Importin-β (IMP-β). Inside the nucleus TN13 could activate defense responses. Whether the Pseudomonas effector HopY1, which was shown to associate with TN13 in a yeast two-hybrid assay, or activation of a basal defense pathway triggers changes in the subcellular localization of TN13 remains to be established. ER, endoplasmic reticulum; PM, plasma membrane; PRR, pattern recognition receptor; TF, transcription factor; TTSS, type III secretion system.