Identification of Small Molecule Ligand Binding Sites On and In the ARNT PAS-B Domain.

Transcription factors are generally challenging to target with small molecule inhibitors due to their structural plasticity and lack of catalytic sites. Notable exceptions to this include a number of transcription factors which are naturally ligand-regulated, a strategy we have successfully exploited with the heterodimeric HIF-2 transcription factor, showing that a ligand-binding internal pocket in the HIF-2α PAS-B domain could be utilized to disrupt its dimerization with its partner, ARNT. Here, we explore the feasibility of directly targeting small molecules to the structurally similar ARNT PAS-B domain, potentially opening a promising route to simultaneously modulate several ARNT-mediated signaling pathways. Using solution NMR screening of an in-house fragment library, we previously identified several compounds that bind ARNT PAS-B and, in certain cases, antagonize ARNT association with the TACC3 transcriptional coactivator. However, these ligands only have mid-micromolar binding affinities, complicating characterization of their binding sites. Here we combine NMR, MD simulations, and ensemble docking to identify ligand-binding 'hotspots' on and within the ARNT PAS-B domain. Our data indicate that the two ARNT/TACC3 inhibitors, KG-548 and KG-655, bind to a ß-sheet surface implicated in both HIF-2 dimerization and coactivator recruitment. Furthermore, KG-548 binds exclusively to the ß-sheet surface, while KG-655 binds to the same site but can also enter a water-accessible internal cavity in ARNT PAS-B. Finally, KG-279, while not a coactivator inhibitor, exemplifies ligands that preferentially bind only to the internal cavity. Taken together, our findings provide a comprehensive overview of ARNT PAS-B ligand-binding sites and may guide the development of more potent coactivator inhibitors for cellular and functional studies.