Molecular Mechanism That Induces Activation of Spätzle, the Ligand for the Drosophila Toll Receptor

Christopher J Arnot,Nicholas J Gay,Monique Gangloff

doi:10.1074/jbc.m109.098186

Christopher J Arnot, Nicholas J Gay + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m109.098186

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Abstract

The Drosophila Toll receptor is activated by an endogenous cytokine ligand Spätzle. Active ligand is generated in response to positional cues in embryonic dorso-ventral patterning and microbial pathogens in the insect immune response. Spätzle is secreted as a pro-protein and is processed into an active form by the serine endoproteases Easter and Spätzle-processing enzyme during dorso-ventral patterning and infection, respectively. Here, we provide evidence for the molecular mechanism of this activation process. We show that the Spätzle prodomain masks a predominantly hydrophobic region of Spätzle and that proteolysis causes a conformational change that exposes determinants that are critical for binding to the Toll receptor. We also gather that a conserved sequence motif in the prodomain presents features of an amphipathic helix likely to bind a hydrophobic cleft in Spätzle thereby occluding the putative Toll binding region. This mechanism of activation has a striking similarity to that of coagulogen, a clotting factor of the horseshoe crab, an invertebrate that has changed little in 400 million years. Taken together, our findings demonstrate that an ancient passive defense system has been adapted during evolution and converted for use in a critical pathway of innate immune signaling and embryonic morphogenesis.

Highlights

In both embryogenesis and immunity, the Spatzle ligand is synthesized as an inactive prepro-protein that is processed internally at the endoplasmic reticulum to remove the N-terminal signal peptide before being secreted from the cell as a homodimer [3, 4]
This suggests that the regions of C-106 required for receptor binding are sequestered by the prodomain and that proteolysis causes a conformational change that exposes the Toll binding surface
We generated 20 models for the C-106. This relates the changes in intensity of the fluorescence emission in the presence and absence of the quenching agent (Q), in this case acrylamide, which causes a decrease in intensity of the emission

Summary

Introduction

In both embryogenesis and immunity, the Spatzle ligand is synthesized as an inactive prepro-protein that is processed internally at the endoplasmic reticulum to remove the N-terminal signal peptide before being secreted from the cell as a homodimer [3, 4]. The Spatzle pro-protein is unable to induce signaling through the Toll pathway but is activated by endoproteolysis. On the basis of these results, we proposed that proteolysis causes a conformational change that exposes the Toll binding sites of Spatzle, but the structural basis for this molecular switch was unclear.

Results

Conclusion