Crystal structure of Arabidopsis thaliana SNC1 TIR domain

Plant immune response is initiated by Resistance proteins (R proteins). Toll/interleukin-1 receptor (TIR) domain in R proteins, which is responsible for the dimerization but has limited conservation in their primary structures. Suppressor of npr1-1, constitutive 1 (SNC1), a TIR-containing R protein, is involved in autoimmunity of plant, but the binding partner of SNC1 via the TIR domain and its specific cognate effector protein remain elusive. Here, we present the crystal structure of the TIR domain of Arabidopsis thaliana SNC1 (AtSNC1-TIR). The structure shows that AtSNC1-TIR domain is similar to those of other plant TIR domains including AtTIR, L6 and RPS4. Structural and sequence analysis on AtSNC1-TIR revealed that almost all conserved amino acids are located in the core of the structure, while the amino acids on the surface are highly variable, implicating that each TIR domain utilizes the variable surface for interacting its binding partner. In addition, the interaction between AtSNC1-TIR proteins in the crystal suggests two possible dimerization modes of AtSNC1-TIR domain. This study provides structural platform to investigate AtSNC1-TIR mediated signaling pathway of plant immune responses.