Three conserved hydrophobic residues in the CC domain of Pit contribute to its plasma membrane localization and immune induction

Abstract

Nucleotide-binding leucine-rich repeat (NLR) proteins work as crucial intracellular immune receptors. N-terminal domains of NLRs fall into two groups, namely coiled-coil (CC) and Toll-interleukin 1 receptor (TIR) domains, which play critical roles in signal transduction and disease resistance. However, the activation mechanisms of NLRs, and how their N-termini are involved in immune induction, remain largely unknown. Here, we revealed that the rice NLR Pit self-associates through its CC domain. The CC domain of Pit possesses three conserved hydrophobic residues that are known to be involved in homodimer formation in two NLRs, barley MLA10 and Arabidopsis RPM1. Interestingly, the function of these residues in Pit is different from that in MLA10 and RPM1. Although the three hydrophobic residues are important for Pit-induced disease resistance against rice blast fungus, they do not participate in self-association or in binding to downstream signaling molecules. Based on homology modeling of Pit using the structure of the Arabidopsis NLR ZAR1, we tried to clarify the role of the three conserved hydrophobic residues and found that they are involved in the plasma membrane localization. Our findings provide novel insights for understanding the mechanisms of NLR activation as well as the relationship between subcellular localization and immune induction.