Journal
JOURNAL OF BIOLOGICAL CHEMISTRY
Volume 284, Issue 6, Pages 3496-3503Publisher
AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
DOI: 10.1074/jbc.M807467200
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- New Zealand's Foundation for Research Science and Technology [UOAX0301, CO6X0301]
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Takeout (To) proteins are found exclusively in insects and have been proposed to have important roles in various aspects of their physiology and behavior. Limited sequence similarity with juvenile hormone-binding proteins (JHBPs), which specifically bind and transport juvenile hormones in Lepidoptera, suggested a role for To proteins in binding hydrophobic ligands. We present the first crystal structure of a To protein, EpTo1 from the light brown apple moth Epiphyas postvittana, solved in-house by the single-wavelength anomalous diffraction technique using sulfur anomalous dispersion, and refined to 1.3 angstrom resolution. EpTo1 adopts the unusual alpha/beta-wrap fold, seen only for JHBP and several mammalian lipid carrier proteins, a scaffold tailored for the binding and/or transport of hydrophobic ligands. EpTo1 has a 45 angstrom long, purely hydrophobic, internal tunnel that extends for the full length of the protein and accommodates a bound ligand. The latter was shown by mass spectrometry to be ubiquinone-8 and is probably derived from Escherichia coli. The structure provides the first direct experimental evidence that To proteins are ligand carriers; gives insights into the nature of endogenous ligand(s) of EpTo1; shows, by comparison with JHBP, a basis for different ligand specificities; and suggests a mechanism for the binding/release of ligands.
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