The Toll/interleukin-1 receptor homology domain is an intracellular signaling domain found in MyD88, interleukin-1 receptors, Toll receptors and many plant R proteins. It contains three highly conserved regions, and mediates protein-protein interactions between the Toll-like receptors and signal-transduction components. TIR-like motifs are also found in plant proteins thought to be involved in resistance to disease. When activated, TIR domains recruit cytoplasmicadaptor proteins MyD88 and TOLLIP. In turn, these associate with various kinases to set off signaling cascades. In Drosophila melanogaster the Toll protein is involved in establishment of dorso-ventral polarity in the embryo. In addition, members of the Toll family play a key role in innate antibacterial and antifungal immunity in insects as well as in mammals. These proteins are type-I transmembrane receptors that share an intracellular 200 residue domain with the interleukin-1 receptor, the Toll/IL-1R homologous region. The similarity between Toll-like receptors and IL-1R is not restricted to sequence homology since these proteins also share a similar signaling pathway. They both induce the activation of a Rel typetranscription factor via an adaptor protein and a protein kinase. MyD88, a cytoplasmic adaptor protein found in mammals, contains a TIR domain associated to a DEATH domain. Besides the mammalian and Drosophila melanogaster proteins, a TIR domain is also found in a number of plant proteins implicated in host defense. As MyD88, these proteins are cytoplasmic. Sitedirected mutagenesis and deletion analysis have shown that the TIR domain is essential for Toll and IL-1R activities. Sequence analysis have revealed the presence of three highly conserved regions among the different members of the family: box 1, box 2, and box 3. It has been proposed that boxes 1 and 2 are involved in the binding of proteins involved in signaling, whereas box 3 is primarily involved in directing localization of receptor, perhaps through interactions with cytoskeletal elements.
