Pf1, a novel PHD zinc finger protein that links the TLE corepressor to the mSin3A-histone deacetylase complex

The mSin3A-histone deacetylase corepressor is a multiprotein complex that is recruited by DNA binding transcriptional repressors. Sin3 has four paired amphipathic alpha helices (PAH1 to -4) that are protein-protein interaction motifs and is the scaffold upon which the complex assembles. We identified a novel mSin3A-interacting protein that has two plant homeodomain (PHD) zinc fingers we term PM, for PHB factor one. PM associates with mSin3A in vivo and recruits the mSin3A complex to repress transcription when fused to the DNA binding domain of Gal4. Pf1 interacts with Sin3 through two independent Sin3 interaction domains (SIDs), Pf1SID1 and Pf1SID2. Pf1SID1 binds PAH2, while Pf1SID2 binds PAH1. Pf1SID1 has sequence and structural similarity to the well-characterized 13-amino-acid SID of the Mad bHLHZip repressor. Pf1SID2 does not have sequence similarity with either Mad SID or Pf1SID1 and therefore represents a novel Sin3 binding domain. Mutations in a minimal fragment of Pf1 that encompasses Pf1SID1 inhibited mSin3A binding yet only slightly impaired repression when targeted to DNA, implying that PM might interact with other corepressors. We show that PM interacts with a mammalian homolog of the Drosophila Groucho corepressor, transducin-like enhancer (TLE). PM binds TLE in an mSin3A-independent manner and recruits functional TLE complexes to repress transcription. These findings suggest that PM may serve to bridge two global transcription networks, mSin3A and TLE.