Binding of the Drosophila cytokine Spätzle to Toll is direct and establishes signaling

Nat Immunol. 2003 Aug;4(8):794-800. doi: 10.1038/ni955. Epub 2003 Jul 20.


The extracellular protein Spätzle is required for activation of the Toll signaling pathway in the embryonic development and innate immune defense of Drosophila. Spätzle is synthesized as a pro-protein and is processed to a functional form by a serine protease. We show here that the mature form of Spätzle triggers a Toll-dependent immune response after injection into the hemolymph of flies. Spätzle specifically bound to Drosophila cells and to Cos-7 cells expressing Toll. Furthermore, in vitro experiments showed that the mature form of Spätzle bound to the Toll ectodomain with high affinity and with a stoichiometry of one Spätzle dimer to two receptors. The Spätzle pro-protein was inactive in all these assays, indicating that the pro-domain sequence, which is natively unstructured, acts to prevent interaction of the cytokine and its receptor Toll. These results show that, in contrast to the human Toll-like receptors, Drosophila Toll requires only an endogenous protein ligand for activation and signaling.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Drosophila / chemistry
  • Drosophila / immunology
  • Drosophila / metabolism*
  • Drosophila Proteins / chemistry
  • Drosophila Proteins / metabolism*
  • Insect Proteins / chemistry
  • Insect Proteins / metabolism*
  • Protein Binding
  • Protein Structure, Tertiary
  • Receptors, Cell Surface / chemistry
  • Receptors, Cell Surface / metabolism*
  • Signal Transduction / physiology*
  • Toll-Like Receptors


  • Drosophila Proteins
  • Insect Proteins
  • Receptors, Cell Surface
  • Tl protein, Drosophila
  • Toll-Like Receptors
  • spz protein, Drosophila