Toll/Interleukin receptor TIR domain superfamily

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 (IL-1R), the Toll/IL-1R homologous region (TIR). The similarity between Toll-like receptors (LTRs) 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 type transcription factor via an adaptor protein and a protein kinase [8621445]. Interestingly, MyD88, a cytoplasmic adaptor protein found in mammals, contains a TIR domain associated to a DEATH domain (see ) [8621445, 9374458, 10679407]. Besides the mammalian and Drosophila melanogaster proteins, a TIR domain is also found in a number of plant proteins implicated in host defense [9868361]. As MyD88, these proteins are cytoplasmic.

Site directed 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 (FDAFISY), box 2 (GYKLC-RD-PG), and box 3 (a conserved W surrounded by basic residues). 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 [10671496].