An essential role for Mad homology domain 1 in the association of Smad3 with histone deacetylase activity

The Smads are a family of sequence-specific DNA-binding proteins that modul ate transcription in response to transforming growth factor beta (TGF beta) by recruiting transcriptional activators like the histone acetyltransferas e, p300/CBP, or repressors like the his tone deacetylase, HDAC1, to TGF bet a target genes. The association of Smads and HDAC1 is mediated in part by d irect binding of Smads to the HDAC1-associated proteins, TG-interacting fac tor, c-ski, and SnoN. Although ectopic expression of these proteins inhibit s Smad-activated transcription, the contribution of histone deacetylase enz ymatic activity to transcriptional repression by TGF beta is unknown. Here, the biological requirements for the interaction between Smads and endogeno us histone deacetylase activity are investigated. We identify residues in M ad homology domain I of Smad3 that are required for association with histon e deacetylase activity. An amino acid change at one of these critical resid ues does not disrupt the association of Smad3 with c-ski, SnoN, and transfo rming growth-interacting factor but does abrogate the ability of Smad3 to r epress transcription, These findings indicate that the association of Smad3 and histone deacetylase activity relies on additional protein mediators th at make contact with Smad3 at its amino terminus. Moreover, these data sugg est that the suppressive effect of Smad3 on transcription is dependent upon its association with histone deacetylase enzymatic activity.