Structure and cooperative formation of a FLI1 filament on contiguous GGAA DNA sites

Ewing sarcoma, a pediatric cancer of bone and soft tissue, is driven in most cases by an abnormal oncogenic fusion of the N-terminal region of EWS with the C-terminal region of FLI1 (EWS-FLI1). The FLI1 region contains a conserved DNA-binding domain (DBD) essential for the oncogenesis. Binding of EWS-FLI1 to microsatellites composed of contiguous GGAA sites, shown previously to be critical for the oncogenic program of this fusion, is not well understood. In this study, we demonstrate that the FLI1 DBD binds cooperatively to contiguous GGAA sites, thereby forming a nucleoprotein filament. A series of crystal structures of two, three, and four FLI1 DBD proteins in complexes with DNA oligomers containing two, three, and four contiguous GGAA sites, respectively, reveal the structure of this filament and the basis for its cooperative formation. We expect this mechanistic insight to be an important milestone in our understanding of the oncogenic function of EWS-FLI1 and exploiting it as a drug target.