Probing the druggability of protein-protein interactions: targeting the Notch1 receptor ankyrin domain using a fragment-based approach

In order to achieve greater selectivity in drug discovery, researchers in both academia and industry are targeting cell regulatory systems. This often involves targeting the protein-protein interactions of regulatory multiprotein assemblies. Protein-protein interfaces are widely recognized to be challenging targets as they tend to be large and relatively flat, and therefore usually do not have the concave binding sites that characterize the so-called 'druggable genome'. One such prototypic nnultiprotein target is the Notch transcription complex, where an extensive network of protein-protein interactions stabilize the ternary complex comprising the ankyrin domain, CSL (CBF1/suppressor of Hairless/Lag-1) and MAML (Mastermind-like). Enhanced Notch activity is implicated in the development of T-ALL (T-cell acute lymphoblastic leukaemia) and selective inhibitors of Notch would be useful cancer medicines. In the present paper, we describe a fragment-based approach to explore the druggability of the ankyrin domain. Using biophysical methods and X-ray crystal structure analyses, we demonstrate that molecules can bind to the surface of the ankyrin domain at the interface region with CSL and MAML. We show that they probably represent starting points for designing larger compounds that can inhibit important protein-protein interactions that stabilize the Notch complex. Given the relatively featureless topography of the ankyrin domain, this unexpected development should encourage others to explore the druggability of such challenging nnultiprotein systems using fragment-based approaches.