The IL-1 receptor accessory protein TIR domain: analysis of putative interaction sites by in-vitro mutagenesis and molecular modeling

Abstract

The Toll/interleukin 1 (IL-1) receptor family plays an important role in both innate and adaptive immunity. These receptors are characterized by a C-terminal homol. motif called the Toll/IL-1 receptor (TIR) domain. A principal function of the TIR domain is mediating homotypic protein-protein interactions in the signal transduction pathway. To suggest interaction sites of TIR domains in the IL-1 receptor complex, the authors modeled the putative three-dimensional structure of the TIR domain within the co-receptor chain, IL-1 receptor accessory protein. The model was based on homol. with the crystal structures of human TLR1 and TLR2. The final structure of the IL-1 receptor accessory protein TIR domain suggests the conserved regions box 1 and 2, including Pro-446, as well as box 3 within the C-terminal a-helix as possible protein-protein interaction sites due to their exposure and their electrostatic potential. Pro-446, corresponding to the Pro/His mutation in dominant neg. TLR4, is located in the third loop at the outmost edge of the TIR domain and does not play any structural role. Inhibition of IL-1 responsiveness seen after substitution of Pro-446 by charged amino acids is due to the loss of an interaction site for other TIR domains. Amino acids 527-534 as part of the loop close to the conserved box 3 are crit. for recruitment of myeloid differentiation factor 88 and to a lesser extent for IL-1 responsiveness. Modeling suggests that native folding of the TIR domain may be approached by the responsive deletion mutants D528-534 and D527-533, whereas the C-terminal b-strand and/or a-helix is displaced in the nonresponsive mutant D527-534.