Phosphorylation of the EWS IQ domain regulates transcriptional activity of the EWS/ATF1 and EWS/FLI1 fusion proteins.

Specific chromosomal translocations are commonly present in mesenchymal tumors and frequently involve genes encoding transcription factors. The combination of different domains from unrelated genes results in chimeric proteins believed to play a key role in the neoplastic process. The EWS/ATF1 and EWS/FLI1 fusion proteins associated with Clear Cell Sarcoma and Ewing's Sarcoma, respectively, were utilized to study the comparative effect of the EWS component on two different DNA binding partners. A potential regulatory site within the EWS IQ domain at serine266 was identified, and studies were performed to demonstrate that EWS is phosphorylated in cells and phosphorylation of serine266 regulates transcriptional activity. Mutational analysis showed that elimination of phosphorylation significantly reduced DNA binding activity by EMSA and reporter activation in luciferase assays, whereas phosphorylation mimicry resulted in a partial restoration to wild-type levels. Phosphorylation was also observed to mediate cellular compartmentalization. These studies confirm that IQ domain phosphorylation regulates the transcriptional activity of exogenous EWS/ATF1 and EWS/FLI1 and suggests that post-translational modifications may potentiate the neoplastic behavior of fusion proteins in general. Since the IQ domain is incorporated into only a subset of fusion transcripts, these findings may provide insight into the molecular mechanism underlying clinical heterogeneity observed in Ewing's sarcoma.