SRE elements are binding sites for the fusion protein EWS-FLI-1

EWS-FLI-1 is a chimeric protein produced in most Ewing's sarcomas. It results from the fusion of the N-terminal-encoding region of the EWS gene to the C-terminal DNA-binding domain (the ETS domain) encoded by the FLI-1 ets family gene. Both EWS-FLI-1 and FLI-1 proteins function as transcription factors that bind specifically to ets sequences (the ets boxes) present in promoter elements. EWS- FLI-1 is a powerful transforming protein, whereas FLI-1 is not. In a search for potential DNA binding sites for these two proteins, we have tested their ability to recognize the serum responsive element (SRE) in the c-fos promoter. This cis element contains an ets box which can be occupied by members of the ETS protein family which do not bind DNA autonomously but form a ternary complex with a second protein, p67SRF (serum responsive factor). We demonstrate here that EWS-FLI-1, but not FLI-1, is able to form a ternary complex on the c-fos SRE. Using a GST pull-down assay, we show that both FLI-1 and EWS-FLI-1 interact in vitro with SRF in the absence of DNA. In electromobility shift assays, EWS-FLI-1 binding to the SRE is detectable in the absence of SRF whereas the binding of FLI-1 is not, suggesting that the interaction with DNA is the step which limits ternary complex formation by FLI-1. Deletion of the N-terminal portion of FLI-1 resulted in a protein which behaved as EWS-FLI-1, suggesting the existence of an N- terminal inhibitory domain in the normal protein. Taken together, our data indicate that there are intrinsic differences in the binding of EWS-FLI-1 and FLI-1 proteins to distinct ets sequences.