MRG15 is a novel chromodomain protein that is a member of a family of genes related to MORF4. MORF4 (mortality factor on chromosome 4) induces senescence in a subset of human tumor cell lines. Our previous results indicated that MRG15 (MORF-related gene on chromosome 15) could derepress the B-myb promoter by association with Rb. In this study, sucrose gradient analysis demonstrated that MRG15 was present in two distinct nuclear protein complexes, MAF1 (MRG15-associated factor 1) and MAF2. Rb was associated with MRG15 and PAM14 (a novel coil-coil protein) in MAF1, and a histone acetyl transferase, hMOF, was an MRG15 partner in MAF2. Analysis of deletion mutants of MRG15 indicated that the leucine zipper at the C-terminal region of MRG15 was important for the protein associations in MAF1 and that the N-terminal chromodomain was required for the assembly of the MAF2 protein complex. Consistent with these data was the fact that a histone acetyltransferase activity associated with MRG15 was lost when the chromodomain was deleted and that both mutant MRG15 proteins failed to activate the B-myb promoter. The various mechanisms by which MRG15 could activate gene transcription are discussed.
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