Toll-like receptors (TLRs) recognize microbial pathogens to trigger an immune response. TLR3, TLR7, and TLR9 recognize various microbial nucleic acids: TLR3, double-stranded RNA; TLR7, single-stranded RNA; and TLR9, unmethylated bacterial DNA. Previously, UNC93B, an endoplasmic reticulum protein predicted to have 12 transmembrane domains, was identified in a genetic screen as essential for signaling by TLR3, TLR7, and TLR9, as well as for antigen presentation by major histocompatibility complexes. Mice carrying the mutation H412R in UNC93B are highly susceptible to infections, and mutations in the encoding gene in humans are associated with susceptibility to viral infections. Brinkmann et al. found that wild-type UNC93B and the H412R mutant exhibited different mobilities upon electrophoresis, but both were produced in similar abundance and appeared to be associated with the endoplasmic reticulum based on their sensitivities to glycosidases. Large-scale isolation of tagged UNC93B or the H412R mutant from RAW macrophages, followed by mass spectrometry of proteins that specifically coprecipitated, revealed that the wild-type protein associated with TLR3, TLR7, TLR9, and TLR13. No TLR peptides were identified in the samples of the H412R mutant. Coexpression of tagged TLR3, TLR4, or TLR9, along with tagged UNC93B, showed that UNC93B selectively interacted with TLR3 and TLR9 but not with TLR4. Chimeras in which the transmembrane domains of TLR4 were exchanged with TLR3 or TLR9 showed that the interaction between UNC93B and TLR3 and TLR7 was mediated by the transmembrane domains. (The H412R mutation is in a predicted transmembrane domain of UNC93B.) To confirm that endogenous UNC93B and TLR7 interacted, the two proteins were coprecipitated from bone marrow-derived dendritic cells. Activation of the TLRs by addition of their respective ligands did not alter the interaction with UNC93B; however, the phenotypes associated with the H412R mutation suggest that these intracellular TLRs require the interaction of UNC93B to function properly. The mechanism by which this endoplasmic reticular protein contributes to TLR signaling remains to be determined.
M. M. Brinkmann, E. Spooner, K. Hoebe, B. Beutler, H. L. Ploegh, Y.-M. Kim, The interaction between the ER membrane protein UNC93B and TLR3, 7, and 9 is crucial for TLR signaling. J. Cell Biol. 177, 265-275 (2007). [Abstract] [Full Text]