A new non-classical fold of varroa odorant-binding proteins reveals a wide open internal cavity

International audienceInsect odorant-binding proteins (OBPs) are not present in other arthropods nor in other living organism. However, ticks, mites, spiders and millipedes contain genes encoding protein with sequence similarity to insect OBPs. In this work we have explored the structure and function of such non-insect OBPs in the mite Varroa destructor, a major pest for honey bees. Varroa OBPs present six cysteines, paired into three disulphide bridges, but their positions in the sequence and their connections are different from those of their insect counterparts. VdesOBP1 structure was determined in two crystal forms closely related and is likely in a monomeric state. Its structure assembles 5 a-helices linked by three disulphide bridges, one of them exhibiting a different connection as compared to their insect counterparts. Comparison with classical OBPs reveals that the second of the six a-helices is lacking in VdesOBP. Ligand-binding experiments revealed molecules able binding only to specific OBPs with a moderate affinity, suggesting that either unknown compounds have to be identified as optimal ligands, or post-translational modifications present in the native proteins may be essential for modulating protein binding activity, or else these OBPs represent a failed attempt in evolution and are not used by the mites