A Silent Partner

Abstract

Like the vertebrate retinoid X receptors (RXR) that it structurally and functionally resembles, the arthropod Ultraspiracle (USP) heterodimerizes with other nuclear receptors to transcriptionally activate target genes. Although various substances activate RXR and have been postulated to serve as endogenous ligands--and the USP binding partner EcR (ecdysone receptor) is known to be activated by ecdysone--the ligand for USP has remained unknown. Noting the similarity between the ligand-binding domain of USP in arthropods that are not Mecopterida (a group of insects) to that of the RXR, Iwema et al . cloned the USP of the beetle Tribolium castaneum (TcUSP) as a representative non-Mecopterida USP. RXR ligands failed to activate a protein containing the TcUSP D/E domains (containing regions important for ligand binding and heterodimerization) fused to the GAL4 DNA-binding domain (GAL4-TcUSP, which acted as a functional partner for EcR) when transiently expressed in cultured human or Drosophila cells. Similarly, RXR ligands failed to activate a GAL4-TcUSP transgene in organ cultures from transgenic Drosophila . Electrospray ionization mass spectrometric analysis indicated that TcUSP failed to bind RXR ligands in vitro, as did surface plasmon resonance (SPR) and limited proteolysis. Analysis of the crystal structure of the TcUSP ligand-binding domain in the context of the functional TcUSP-EcR heterodimer indicated that TcUSP exhibited a stable apo structure in an inactive "antagonist" conformation, which did not have a conventional ligand-binding pocket; moreover, SPR revealed that it failed to bind coactivator peptides. Phylogenetic analysis emphasized the evolutionary plasticity of the RXR-USP family ligand-binding domain, and indicated a lower evolutionary rate for the ligand-binding region of the vertebrate RXR than for that of the non-Mecopterida USP, suggesting that, even though non-Mercopterida USP do not, RXR do indeed bind endogenous ligands. T. Iwema, I. M. Billas, Y. Beck, F. Bonneton, H. Nierengarten, A. Chaumot, G. Richards, V. Laudet, D. Moras, Structural and functional characterization of a novel type of ligand-independent RXR-USP receptor. EMBO J. 26 , 3770-3782 (2007). [PubMed]