Identification and Characterization of a Novel Nuclear Protein Complex Involved in Nuclear Hormone Receptor-mediated

with NRC.Although NIF-1 does not directly interact with nuclear hor-mone receptors, it enhances activation by nuclear hormonereceptorspresumablythroughitsinteractionwithNRC.Tofur-ther understand the cellular and biological function of NIF-1,we identified NIF-1-associated proteins by in-solution proteol-ysisfollowedbymassspectrometry.Theidentifiedcomponentsrevealed factors involved in histone methylation and cell cyclecontrolandincludeAsh2L,RbBP5,WDR5,HCF-1,DBC-1,andEMSY. Although the NIF-1 complex contains Ash2L, RbBP5,and WDR5, suggesting that the complex might methylate his-toneH3-Lys-4,wefoundthatthecomplexcontainsaH3meth-yltransferase activity that modifies a residue other thanH3-Lys-4. The identified components form at least two dis-tinctly sized NIF-1 complexes. DBC-1 and EMSY were identi-fied as integral components of an NIF-1 complex of 1.5 MDaand were found to play an important role in the regulation ofnuclear receptor-mediated transcription. Stimulation of the