Editors' ChoiceReceptors

Allosteric Activation of Toll-Like Receptor Activity

Science's STKE  03 Jul 2007:
Vol. 2007, Issue 393, pp. tw239
DOI: 10.1126/stke.3932007tw239

One of the most fundamental questions in signal transduction--precisely how receptors transmit information across the cell membrane--is not well understood. Latz et al. addressed this problem by analyzing signaling by a receptor of the innate immune system, Toll-like receptor 9 (TLR9), and obtained some unexpected results. TLR9 recognizes the presence of invading microbes by detecting microbial DNA, which is relatively enriched in unmethylated CpG motifs. Not all CpG-containing DNA fragments are stimulatory to the TLR9 receptor, and DNA with GGGG sequences can actually inhibit receptor signaling. However, the authors found that stimulatory, inhibitory, and control DNA fragments bound TLR4 with similar affinities. Thus, simple ligand binding appears not to elicit receptor activation. When the authors used circular dichroism spectroscopy to analyze conformational changes in a fusion protein of the TLR ectodomain with the Fc fragment of immunoglobulin G, they found that only stimulatory DNA induced a conformation change in the receptor fusion protein. Further evidence for a change in the proximity of the cytoplasmic signaling portion of the receptor upon binding of stimulatory DNA in living cells was obtained in experiments monitoring fluorescence resonance energy transfer (FRET). For these experiments, the authors tagged full-length TLR9 molecules on their C termini with appropriate donor and acceptor molecules. These studies also showed that TLR9 existed as a preformed dimer before the receptor bound DNA. The authors also measured behavior of the cytoplasmic signaling side of the receptor by creating two fusion proteins that each expressed only half of the green fluorescent protein. In cells expressing both proteins, fluorescence (attributable to folding of the two GFP fragments, thus indicating very close apposition of the two receptor proteins) was observed only in the presence of stimulatory CpG-containing DNA and only in the endosomes. Together, the results indicate that allosteric changes induced by binding of stimulatory DNA, rather than ligand-induced oligomerization, is the key to activation of TLR9. Thus, the authors note, it may be possible to more specifically select useful therapeutic agents by focusing on inhibition of the activating allosteric changes rather than inhibition of ligand binding. Such agents would be welcome because chronic overactivation of TLRs leads to autoimmune disorders that result from excessive inflammatory responses initiated by these receptors. Kim et al. discuss the results.

E. Latz, A. Verma, A. Visintin, M. Gong, C. M. Sirois, D. C. G. Klein, B. G. Monks, C. J. McKnight, M. S. Lamphier, W. P. Duprex, T. Espevik, D. T. Golenbock, Ligand-induced conformational changes allosterically activate Toll-like receptor 9. Nat. Immunol. 8, 772-779 (2007). [PubMed]

Y.-M. Kim, M. M. Brinkmann, H. L. Ploegh, TLRs bent into shape. Nat. Immunol. 8, 675-677 (2007). [PubMed]