Abstract

Drosophila Toll plays a critical role in both embryonic development and innate immunity in response to an endogenous protein ligand called Spätzle (Spz). Evolutionary-related Toll-like receptors also function in immunity but are activated directly by pathogen-associated molecules such as bacterial endotoxin. We determined the crystal structure of dimeric Spz bound to a fragment of the Toll ectodomain encompassing the first 13 leucine-rich repeats at a resolution of 2.35-Å. The cystine-knot domains of Spz bind the concave face of the Toll leucine-rich repeat solenoid in a 1:1 complex delineated by N-linked glycans. The binding mode is reminiscent of nerve growth factor (NGF) in complex with neurotrophin receptor p75NTR. Despite ellipsoidal truncation and anisotropic scaling of the diffraction data that significantly improved electron density maps, the two apical β-wings and Trp loops of Spz could not be resolved raising the question of their implication in the mechanism of negative cooperativity in Toll signalling.