Three novel mammalian Toll-like receptors: gene structure, expression, andevolution

We describe three novel genes, encoding members of the Toll-like receptor ( Tlr) family (TLR7, TLR8, and TLR9), These Tlr family members, unlike others reported to date, were identified within a genomic database. TLR7 and TLR8 each have three exons, two of which have coding function, and lie in close proximity to one another at Xp22, alongside a pseudogene, The remaining ge ne (TLR9) resides at 3p21.3 (in linkage with the MyD88 gene), and is expres sed in at least two splice forms, one of which is monoexonic and one of whi ch is biexonic, the latter encoding a protein with 57 additional amino acid s at the N-terminus. The novel Tlrs comprise a cluster as nearest phylogene tic neighbors. Combining all sequence data related to Toll-like receptors, we have drawn several inferences concerning the phylogeny of vertebrate and invertebrate Tlrs. According to our best estimates, mammalian TLRs 1 and 6 diverged from a common mammalian ancestral gene 95 million years ago. TLR4 , which encodes the endotoxin sensor in present-day mammals, emerged as a d istinct entity 180 million years ago. TLRs 3 and 5 diverged from a common a ncestral gene approximately 150 million years ago, as did Tlr7 and TIr8, Ve ry likely, fewer Tlrs existed during early vertebrate evolution: at most th ree or four were transmitted with the primordial vertebrate line. Phylogene tic data that we have adduced in the course of this work also suggest the e xistence of a Drosophila equivalent of MyD88, and indicate that the plasma membrane protein SIGIRR is close functional relative of MyD88 in mammals. F inally, a single present-day representative of the Toll-like proteins in Dr osophila has striking cytoplasmic domain homology to mammalian Tlrs within the cluster that embraces TLRs 1, 2, 4, and 6, This would suggest that an a ncestral (pre-vertebrate) Tlr may have adopted a pro-inflammatory function 500 million years ago.