Dimerization of Toll-like receptor 3 (TLR3) is required for ligand binding

TLR3 (Toll-like receptor 3) recognizes dsRNA, a potent indicator of viral infection. The extracellular domain of TLR3 dimerizes when it binds dsRNA, and the crystal structure of the dimeric complex reveals three sites of interaction on each extracellular domain, two that bind dsRNA and one that is responsible for dimer formation. The goal of this study was to determine which amino acid residues are essential for forming a stable receptor·ligand complex and whether dimerization of TLR3 is required for dsRNA binding. Using a novel ELISA to analyze dsRNA binding by mutant TLR3 constructs, we identified the essential interacting residues and determined that the simultaneous interaction of all three sites is required for ligand binding. In addition, we show that TLR3 is unable to bind dsRNA when dimerization is prevented by mutating residues in the dimerization site or by immobilizing TLR3 at low density. We conclude that dimerization of TLR3 is essential for ligand binding and that the three TLR3 contact sites individually interact weakly with their binding partners but together form a high affinity receptor·ligand complex.