Dynamic of ligand binding to Drosophila melanogaster ecdysteroid receptor

Ligand binding to ecdysone receptor (EcR) is an autonomous function of the ligand binding domain (LBD) and is not modified by other receptor domains or tags fused to the LBD. Association and dissociation velocity of hormone to EcR was studied in the absence and presence of its main dimerization partner Ultraspiracle (USP). Mutational analysis of the EcR(LBD) revealed that ligand entry and exit is affected differently by the same point mutation, indicating that different pathways are used for association and dissociation of the ligand. Heterodimerization with wild type USP(LBD) increases ligand association to EcR(LBD) about fivefold and reduces dissociation 18-fold. Opposite effects of the same mutation (N626K) on dissociation velocity of ligand in EcR and EcR/USP indicate that not only hormone binding itself, but also the kinetic behaviour of ligand binding is modified by the dimerization partner. A general effect of the point mutations on the 3D architecture seems unlikely due to the highly selective effects on the kinetics of hormone binding.