The mammalian ATM/PI 3-kinase-related TRRAP protein was previously found to be a component of a multiprotein histone acetyltransferase (HAT) complex containing the HAT TIP60. In this report, we identify a previously uncharacterized protein encoded by the FLJ10914 ORF, which we designate MRGBP, as a new component of the TRRAP/TIP60 HAT complex. In addition, through purification of MRGBP and its associated proteins from HeLa cell nuclear extracts, we identify the thyroid receptor coactivating protein (TRCp120), DMAP1, and the related MRG15 and MRGX proteins as MRGBP-associating proteins, and we present biochemical evidence that they are previously unrecognized components of the TRRAP/TIP60 HAT complex. Taken together, our findings shed new light on the structure and function of the mammalian TRRAP/TIP60 histone acetyltransferase complex.The ATM/PI 3-kinase-related TRRAP protein is an integral subunit of multiple distinct histone acetyltransferase (HAT) 1 complexes that have critical roles in transcriptional regulation. To date, the best characterized TRRAP-containing HAT is the Saccharomyces cerevisiae SAGA complex (1-3). The SAGA complex includes the yeast TRRAP homolog Tra1, the Ada1, Ada2, and Ada3 proteins, the Spt3, Spt7, Spt8, and Spt20 proteins, several TAFs including TAF5, TAF6, TAF9, TAF10, and TAF12, and Gcn5, the founding member of the Gcn5-related N-acetyltransferase (GNAT) superfamily of HATs.In contrast to yeast, mammalian cells contain multiple TRRAP-containing SAGA-like HATs including the STAGA (SPT3-TAF(II)31-Gcn5-L acetyltransferase), PCAF (p300/CBPassociated factor), and TFTC (TATA-binding protein-free TAF(II) complex) complexes. These GNAT HATs have overlapping but non-identical subunit compositions. The STAGA complex includes TRRAP, mammalian homologs of the yeast Ada1 (STAF42), Ada2, and Ada3 proteins, mammalian homologs of the yeast Spt3 and Spt7 proteins, TAF9 and TAF12, PAF65␣, and PAF65, which are similar to TAF6 and TAF5, respectively, the splicing factor SAP130, the UV-damaged DNA-binding protein DDB1, and a mammalian homolog of the yeast Gcn5 HAT referred to as Gcn5L (4, 5). The PCAF complex includes TRRAP, mammalian homologs of the yeast Ada2, Ada3, and Spt3 proteins, TAF9 and TAF12, the TAF-like proteins PAF65␣ and PAF65, and the Gcn5-related HAT PCAF (6). Finally, the TFTC complex includes TRRAP, the mammalian Ada2 and Spt3 proteins, TAF2, TAF4, TAF5, TAF6, TAF7, TAF9, TAF10, and TAF12, the PAF65␣ and PAF65 proteins, SAP130, and Gcn5 (7-9)}.In addition to its role as a subunit of the S. cerevisiae SAGA and mammalian SAGA-like complexes, the TRRAP protein is also a component of a structurally unrelated HAT referred to in yeast as the NuA4 complex. The S. cerevisiae NuA4 complex includes TRRAP homolog Tra1, the Enhancer of Polycomb protein, the chromodomain protein Eaf3, p33ING1 tumor suppressor homolog Yng2, actin-like protein Arp4, actin, and Esa1, a HAT belonging to the MYST family (10 -13).A multiprotein NuA4-like HAT has also been identified in mammalian cells and, like the...