Toll-like receptors (TLRs) are key receptors for the activation of immune responses directed against pathogens. Among the more than 10 identified TLRs, TLR4 is the most unique because it associates with a variety of adaptor molecules for ligand recognition and signal transduction. However, the relationship between the unique characteristics and structural features of TLR4 is poorly defined. In this study, we demonstrate a novel biochemical characteristic of TLR4. TLR4, but not other TLRs, was observed as highly aggregated forms in immunoblotting. Interestingly, substitution of the transmembrane and cytoplasmic domain of TLR4 with those of other TLRs completely abolished the aggregation of TLR4. Furthermore, we found a short hydrophobic region (HR) adjacent to the transmembrane domain of TLR4; the TLR4 mutant lacking the HR was not aggregated and was nonfunctional in response to lipopolysaccharide. These results suggest that the HR may play a critical role in the functional oligomerization of TLR4. (c) 2006 Elsevier Inc. All rights reserved.