Critical Role of Desolvation in the Binding of 20-Hydroxyecdysone to the Ecdysone Receptor

Abstract

The insect steroid hormone 20-hydroxyecdysone (20E) binds to its cognate nuclear receptor composed of the ecdysone receptor (EcR) and Ultraspiracle (USP) and triggers the main developmental transitions, in particular molting and metamorphosis. We present the crystal structure of the ligand-binding domains of EcR/USP in complex with 20E at 2.4 Å resolution and compare it with published structures of EcR/USP bound to ponasterone A (ponA). ponA is essentially identical to 20E but lacks the 25-OH group of 20E. The structure of 20E-bound EcR indicates that an additional hydrogen bond is formed compared with the ponA-bound receptor, yet, paradoxically, ponA has a significantly higher affinity for EcR than 20E.

The palindromic DNA-bound USP/EcR nuclear receptor adopts an asymmetric organization with allosteric domain positioning

“…2a). The LBDs have their N-terminal end oriented towards the DNA, whereas the ligandbinding pockets 13,15 are oriented away from the DNA (Fig. 2a).…”

Semi‐Synthetic Ecdysteroids as Gene‐Switch Actuators: Synthesis, Structure–Activity Relationships, and Prospective ADME Properties

“…Potency is retained or improved by differ- [68] Consistent with an H-bond acceptor assignment is also the observation that steroids lacking a 22-OH group altogether are substantially depleted in potency. [33] Available crystal structures of PoA bound to Tribolium castaneum EcR [31] and 20E bound to HvEcR [32] indicate water-mediated H-bridges between 22-OH and a conserved Asp residue. The observations neither obligate an Hbond donor role nor preclude an H-bond acceptor role for 22-OH.…”

“…Computational studies of EcR binding of PoA and 20E have invoked differential desolvation energies to explain superior receptor affinity for PoA. [32] Subtle energy adjustments of ligand desolvation and water tally in the LBD may account for unexpected inversions in potency of ecdysteroids possessing non-H-bonding vs. H-bonding groups at the 25-position.…”

“…The half-life in humans is estimated to be as low as 4 h (ecdysone) and 9 h (20E). [28] Additionally, the specific H-bond acceptor/H-bond donor role of each of these hydroxy groups in EcR binding and geneswitch potency is not entirely transparent, in spite of the availability of crystal structures of several ligand-EcR complexes, [29][30][31][32] as well as multi-dimensional QSAR modelling. [33,34] Herein we report the first systematic synthetic exploration of ecdysteroids toward modulation of gene-switch potency.…”

The Structure and Function of Ecdysone Receptors

“…The structure of the H. virescens EcR/USP LBD in complex with the natural ecdysteroid hormone (20E) has recently also been reported (Browning et al, 2007). The overall heterodimeric arrangement of these EcR/USP LBDs is closely similar, with each LBD subunit exhibiting, as anticipated, the For H. armigera, this axis also represents 0.3x the concentration of competing ligand used with the E/F heterodimer (the resulting horizontal displacement compensates for the poorer binding obtained with the E/F heterodimer and renders the E/F and DE/F curves coincident).…”