HDA2 and HDA3 are related proteins that interact with and are essential for the activity of the yeast histone deacetylase HDA1

Histone deacetylase HDA1, the prototype for the class II mammalian deacetyl ases, is likely the catalytic subunit of the HDA1-containing complex that i s involved in TUP1-specific repression and global deacetylation in yeast. A lthough the class I RPD3-like enzymatic complexes have been well characteri zed, little is known about the identity and interactions of the factors tha t associate to form the HDA1 complex. In this paper, we identify related HD A2 and HDA3 proteins that are found in the HDA1 complex and show that HDA1 interacts with itself and with the HDA2-HDA3 subcomplex to form a likely te tramer. These interactions are necessary for catalytic activity because mut ations in any of the three components disrupt activity both in vitro and in vivo. In this respect the HDA1 complex differs from yeast RPD3, which has components such as SIN3 that are not essential for activity in vitro, and y east HOS3, which has intrinsic in vitro activity as a homodimer in the abse nce of other subunits.