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 Å resolution. EpTo1 adopts the unusual ␣/-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 Å 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. Takeout (To) 3 family have been proposed to have important roles in various aspects of insect physiology and behavior and as such may represent novel targets for insect control strategies. The original member, To, was discovered in Drosophila melanogaster as a clock-regulated gene, acting as a molecular link between circadian rhythms and feeding behavior (1, 2). In addition, To expression is regulated by the somatic sex determination pathway and affects male courtship behavior (3, 4). To has a predicted secretion signal (1) and is secreted in the hemolymph of adult males (4). At least 80 To genes have now been identified from seven orders of insect, including 20 from D. melanogaster, but few have been characterized in any detail. Due to the localization and/or expression profiles of their transcripts, some are proposed to have roles in chemosensory perception (5-8), whereas others are thought to be involved in binding terpenoids (9), including juvenile hormones (JH) (10 -15).
Members of theTo proteins share limited sequence similarity and a conserved N-terminal disulfide bond with juvenile hormone-binding proteins (JHBPs). These proteins specifically bind and transport JH to target tissues in the Lepidoptera, further suggesting a role for To proteins in binding hydrophobic ligands (2). There are, however, two major differences between the families: a second disulfide bond in the JHBPs, not present in To; and the presence of two conserved C-terminal sequence motifs that are unique to the To family (2). Very recently, the structure of JHBP from the moth Galleria mellonella (GmJHBP) has been solved, revealing an unusual fold in which a highly curved antiparallel -sheet wraps around a...