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The Drosophila Toll receptor, which functions in both embryonic patterning and innate immunity to fungi and Gram-positive bacteria, is activated by a dimeric cytokine ligand, Spätzle (Spz). Previous studies have suggested that Spz crosslinks two Toll receptor molecules to form an activated complex. Here we report electron microscopy structures of the Toll ectodomain in absence and in presence of Spz. Contrary to expectations, Spz does not directly crosslink two Toll ectodomains. Instead Spz binding at the N-terminal end of Toll predominantly induces the formation of a 2:2 complex, with two sites of interaction between the ectodomain chains, one located near to the N-terminus of the solenoid, the other between the C-terminal juxtamembrane sequences. Moreover Toll undergoes a ligand-induced conformational change, becoming more tightly curved than in the apo form. The unexpected 2:2 complex was confirmed by mass spectrometry under native conditions. These results suggest that activation of Toll is an allosteric mechanism induced by an end-on binding mode of the ligand.

Structural insight into the mechanism of activation of the Toll receptor