Functional insights from the crystal structure of the N-terminal domain of the prototypical toll receptor.

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Arnot, Christopher J 
Lewis, Miranda 
Gay, Nicholas J 

Drosophila melanogaster Toll is the founding member of an important family of pathogen-recognition receptors in humans, the Toll-like receptor (TLR) family. In contrast, the prototypical receptor is a cytokine-like receptor for Spätzle (Spz) protein and plays a dual role in both development and immunity. Here, we present the crystal structure of the N-terminal domain of the receptor that encompasses the first 201 amino acids at 2.4 Å resolution. To our knowledge, the cysteine-rich cap adopts a novel fold unique to Toll-1 orthologs in insects and that is not critical for ligand binding. However, we observed that an antibody directed against the first ten LRRs blocks Spz signaling in a Drosophila cell-based assay. Supplemented by point mutagenesis and deletion analysis, our data suggests that the region up to LRR 14 is involved in Spz binding. Comparison with mammalian TLRs reconciles previous contradictory findings about the mechanism of Toll activation.

Amino Acid Motifs, Amino Acid Sequence, Animals, Antibodies, Cell Line, Crystallography, X-Ray, Drosophila Proteins, Drosophila melanogaster, Models, Molecular, Molecular Sequence Data, Protein Binding, Protein Stability, Protein Structure, Secondary, Protein Structure, Tertiary, Recombinant Fusion Proteins, Sequence Homology, Amino Acid, Signal Transduction, Toll-Like Receptors
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Elsevier BV
Medical Research Council (G1000133)
Wellcome Trust (100321/Z/12/Z)
This work is financed by the Wellcome Trust Award (RG47206). We thank Dr. Martin Moncrieffe for helpful discussions and Ms. Irina Ogay from the Baculovirus Facility, Department of Biochemistry, Cambridge, for protein expression. We are grateful to Dr. Katherine Stott, from the Biophysics Facility, for her aid with analytical ultracentrifugation. Thanks to Dr. Dimitri Chirdgaze, from the Crystallographic X-ray Facility, for his assistance. We are grateful to Prof. Abel Moreno for help with capillary crystallization. We thank the staff at beamlines IO3 at Diamond Light Source, England, and ID23EH1 at the ESRF, Grenoble, France, for help with the data collection.
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