Members of the Mad family of bHLHZip proteins heterodimerize with Max and function to repress the transcriptional and transforming activities of the Myc protooncogene. Mad:Max heterodimers repress transcription by recruiting a large multi-protein complex containing the histone deacetylases, HDAC1 and HDAC2, to DNA. The interaction between Mad proteins and HDAC1/2 is mediated by the corepressor mSin3A and requires sequences at the amino terminus of the Mad proteins, termed the SID, for Sin3

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... action domain, and the second of four paired amphipathic ␣-helices (PAH2) in mSin3A. To better understand the requirements for the interaction between the SID and PAH2, we have performed mutagenesis and structural studies on the SID. These studies show that amino acids 8 -20 of Mad1 are sufficient for SID:PAH2 interaction. Further, this minimal 13-residue SID peptide forms an amphipathic ␣-helix in solution, and residues on the hydrophobic face of the SID helix are required for interaction with PAH2. Finally, the minimal SID can function as an autonomous and portable repression domain, demonstrating that it is sufficient to target a functional mSin3A/HDAC corepressor complex.