Molting hormone receptor (EcR/USP) / steroidal ligand ponA complex
Heliothis virescens (noctuid moth)
Every year insects cause major ravages to crops . As early as the late 19-th century chemicals have been used in the fight against them . In the beginning almost all of the insecticides were inorganic substances, but since the discovery of the infamous DDT in the mid-20th century, thousands of new organic compounds have been synthesized in order to find better agents. But after years of usage of chemical insecticides global concerns have been raised against their application . They are damaging the ecological biosystems, poisoning people and other organisms, while at the same time insects are developing resistance towards them.
The drawbacks of chemical insecticides stimulated the search for new forms of control. Biologically active compounds seem a promising target for development due to their specificity and few if any harmful side effects. One major group of biological insecticides are hormones which can be used to control or disrupt such vital processes as pests' growth and reproduction.
Several crucial phases of insect development, like moulting and metamorphosis, are controlled by ecdysteroid hormones. The hormones need to bind to a specific receptor, called ecdysteroid receptor (EcR), in order to perform their function. The biologically active form of EcR is an ultra-spiracle (USP) bound heterodimer.
The structure presented here shows the heterodimer in complex with BYI06830, a non-steroidal insecticide. The general framework of the EcR and USP ligand binding domains (LBDs) is formed of a β-sheet and three layers of α-helices oriented anti-parallel to each other. A long peptide loop that connects Helix5 and the beta-sheet in USP is positioned at the interface of the two domains. It is assumed to have an important role in function regulation.
But most unusual is the structure of the ligand binding pocket. During the complex' formation with BYI06830, the interactions between the second and third beta-sheets are disrupted. This leads to the creation of the bulky V-shaped cavity in order to accommodate the chemical. When we compare it to the structure of EcR-LBD bound to ecdysteroid ponasteronA we can see that the shape is very different – the ponasteronA cavity is long and thin, L-shaped and completely buried inside the receptor. This means that the protein is able to adopt different pocket shapes depending on the structure and shape of the ligand. Thus, the flexibility of EcR-LDB can be explored with different compounds, and presents us with new aspects for drug design.
Protein Data Bank (PDB)
author: Rossen Apostolov