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Brachyury is a transcription factor that plays an essential role in tumour growth of the rare bone cancer chordoma and is implicated in other solid tumours. Brachyury is minimally expressed in healthy tissues, making it a potential therapeutic target. Unfortunately, as a ligandless transcription factor, brachyury has historically been considered undruggable. To investigate direct targeting of brachyury by small molecules, we determine the structure of human brachyury both alone and in complex with DNA. The structures provide insights into DNA binding and the context of the chordoma associated G177D variant. We use crystallographic fragment screening to identify hotspots on numerous pockets on the brachyury surface. Finally, we perform follow-up chemistry on fragment hits and describe the progression of a thiazole chemical series into binders with low µM potency. Thus we show that brachyury is ligandable and provide an example of how crystallographic fragment screening may be used to target protein classes that are difficult to address using other approaches. This study describes structures of the transcription factor brachyury revealing the mechanism of DNA recognition. They identify fragments using X-ray fragment screening and optimize these into potent ligands with potential as cancer therapeutics.

The SARS-CoV-2 non-structural protein 14 (NSP14) is a dual function enzyme containing an N-terminal exonuclease domain (ExoN) and C-terminal Guanine-N7-methyltransferase (N7-MTase) domain. Both enzymatic activities appear to be essential for the viral life cycle and thus may be targeted for anti-viral therapeutics. NSP14 forms a stable complex with the SARS-CoV-2 zinc binding protein NSP10, and this interaction greatly enhances the nuclease but not the methyltransferase activity. In this study, we have determined the crystal structure of SARS-CoV-2 NSP14 in the absence of NSP10 to 1.7 Å resolution. Comparisons of this structure with the structure of NSP14/NSP10 complexes solved to date reveal significant conformational changes that occur within the NSP14 ExoN domain upon binding of NSP10, including significant movements and helix to coil transitions that facilitate the formation of the ExoN active site and provide an explanation of the stimulation of nuclease activity by NSP10. Conformational changes are also seen in the MTase active site within a SAM/SAH interacting loop that plays a key role in viral mRNA capping. We have also determined the structure of NSP14 in complex with cap analogue 7MeGpppG, offering new insights into MTase enzymatic activity. We have used our high resolution crystals to perform X-ray fragment screening of NSP14, revealing 72 hits bound to potential sites of inhibition of the ExoN and MTase domains. These structures serve as excellent starting point tools for structure guided development and optimization of NSP14 inhibitors that may be used to treat COVID-19 and potentially other future viral threats. Competing Interest Statement The authors have declared no competing interest.

The transcription factor brachyury is a member of the T-Box family of transcription factors. It is active during embryogenesis and is required for the formation of the posterior mesoderm and the notochord in vertebrates. Aside from its role in embryogenesis, brachyury plays an essential role in tumour growth of the rare chordoma bone cancer and is implicated in other solid tumours. Given that brachyury is minimally expressed in healthy tissues, these findings suggest that brachyury is a potential therapeutic target in cancer. Unfortunately, as a ligandless transcription factor, brachyury has historically been considered undruggable. To investigate direct targeting of brachyury by small molecules, we initially determined the structure of human brachyury both in complex with its cognate DNA and in the absence of DNA. Analysis of these structures provided insights into brachyury DNA binding and the structural context of the G177D variant which is strongly associated with chordoma risk. We used these structures to perform a crystallographic fragment screen of brachyury and identify hotspot regions on numerous pockets on the brachyury surface. Finally, we have performed follow-up chemistry on fragment hits and describe the structure-based progression of a thiazole-containing chemical series. Excitingly, we have produced brachyury binders with low µM potency that can serve as starting point for further medicinal chemistry efforts. These data show that brachyury is ligandable and provides an example of how crystallographic fragment screening may be used to find ligands to target protein classes that are traditionally difficult to address using other approaches.