AbstractBACKGROUNDU19/Eaf2, an androgen‐response gene, is downregulated in advanced human prostate cancer specimens and its overexpression can markedly induce apoptosis in prostate cancer cells. Eleven‐nineteen Lysine‐rich Leukemia (ELL) is an RNA polymerase II transcription elongation factor, initially identified as a fusion partner gene of MLL in the t(11; 19) (q23; p13.1) chromosomal translocation in acute myeloid leukemia. U19/Eaf2 was previously reported as an ELL‐associated factor, a potential transcription factor binds to ELL, forming nuclear speckles in vivo. These findings suggest that ELL–U19/Eaf2 interaction is potentially important in prostate cancer progression and/or acute myeloid leukemia. However, the functional significance of U19/Eaf2 interaction with ELL remains unclear.METHODSUsing co‐transfection, co‐immunoprecipitation, protein stability assay and transactivation assay, we characterized the consequence of ELL binding to U19/Eaf2.RESULTSWe provide further evidence for U19/Eaf2 as a transcription factor and show that ELL binding is required for nuclear speckle formation of human U19/Eaf2, stabilizes U19/Eaf2 and enhances its transactivation activity.CONCLUSIONSThe above observations indicate ELL may be an important factor required for U19/Eaf2 function because U19/Eaf2 nuclear localization and transactivation activity are essential for its function as a transcription factor. Published 2005 Wiley‐Liss, Inc.