Plants respond to pathogens with a response called the hypersensitive response that involves necrotic cell death of the infected tissue to prevent further infection. Mounting this response involves the recognition of pathogen elicitor proteins and the plant products of the Resistance (R) genes, but the exact mechanism remains unclear. Burch-Smith et al. used the p50 helicase domain of the Tobacco mosaic virus replicase protein to study in plants the interaction between this p50 protein elicitor and the product of the N gene, which encodes an R protein with a leucine-rich repeat (LRR), a nucleotide-binding (NB) site, and a Toll-interleukin-1 receptor (TIR) homolog domain. Plant tissue in which a tagged version of p50 and a tagged version of N were coexpressed exhibited the hypersensitive response. Biochemical fractionation indicated that both proteins were soluble, and fluorescence analysis indicated that both proteins could be found in the cytoplasm and the nucleus of infected leaf cells. Inhibition of the nuclear localization of the N protein through the addition of a nuclear export sequence prevented the hypersensitive response, whereas blocking nuclear localization of p50 did not. This suggests that N and p50 interact in the cytoplasm but that N has nuclear functions that are required for activation of the immune response. Coimmunoprecipitation studies with the tagged proteins, as well as bimolecular fluorescence complementation studies, indicated that the two proteins interacted. However, purified proteins did not interact in vitro, which suggests that the interaction is indirect and that there is another molecule required for p50 and N interaction. Although the LRR domain is best known for recognition of pathogen sequences, the TIR domain of N appeared to be necessary and sufficient for the interaction of p50 with N. Deletion or point mutation of the TIR domain blocked the interaction between the two proteins. The isolated TIR domain of N, but not that of other R proteins, interacted with p50. The authors propose that the TIR domain of N (through an unknown adaptor) recognizes p50, which leads to a change in the N protein, and that the "activated" N protein then engages the defense response through a process that requires the nuclear localization or activation of a nuclear pool of N.
T. M. Burch-Smith, M. Schiff, J. F. Caplan, J. Tsao, K. Czymmek, S. P. Dinesh-Kumar, A novel role for the TIR domain in association with pathogen-derived elicitors. PLoS Biol. 5, e68 (2007). [PubMed]