Histone deacetylases (HDACs) are protein deacetylases that play a role in repression of gene transcription and are emerging targets in cancer therapy. Here, we characterize the structure and enzymatic activity of the catalytic domain of human HDAC7 (cdHDAC7). Although HDAC7 normally exists as part of a multiprotein complex, we show that cdHDAC7 has a low level of deacetylase activity which can be inhibited by known HDAC inhibitors. The crystal structures of human cdHDAC7 and its complexes with two hydroxamate inhibitors are the first structures of the catalytic domain of class IIa HDACs and demonstrate significant differences with previously reported class I and class IIb-like HDAC structures. We show that cdHDAC7 has an additional class IIa HDAC-specific zinc binding motif adjacent to the active site which is likely to participate in substrate recognition and protein-protein interaction and may provide a site for modulation of activity. Furthermore, a different active site topology results in modified catalytic properties and in an enlarged active site pocket. Our studies provide mechanistic insights into class IIa HDACs and facilitate the design of specific modulators.The level of histone acetylation is regulated by the action of two classes of enzymes, histone acetyltransferases and histone deacetylases (HDACs).3 Histone acetyltransferases and HDACs are found in large multiprotein complexes, and recruitment of histone acetylase or deacetylase complexes by coactivators or corepressors is thought to cause a local change in the chromatin structure, resulting in either activation or repression of gene transcription (1). Humans have 18 HDACs and, based on their sequence similarity to yeast factors, they are grouped into four classes (class I-IV). Class II HDACs are homologous to yeast histone deacetylase HDA1 and have been implicated as global regulators of gene expression during cell differentiation and development (2). In humans, class II HDACs are subdivided into classes IIa (HDAC4, HDAC5, HDAC7, and HDAC9) and IIb (HDAC6 and HDAC10). Class IIa HDACs contain two functionally important regions, a highly conserved C-terminal catalytic domain and an N-terminal extension that has no similarity with other proteins, mediates the signal-dependent shuttling between the nucleus and the cytoplasm, and harbors binding sites for transcriptional regulators (2, 3). Class IIa HDACs interact with corepressors such as N-CoR (nuclear receptor corepressor) and the MEF2 (myocyte enhancer factor 2) family of transcription factors that is not only important for controlling gene expression in normal cellular programs like muscle differentiation, T-cell apoptosis, neuronal survival, and synaptic differentiation but has also been linked to cardiac hypertrophy, asthma, atherosclerosis, hypertension, and other pathological conditions (3-5). To date all four class IIa HDACs have been knocked out in mice, and the resulting abnormal phenotypes have been extensively characterized (6 -9). HDAC7 for example, plays an important role in cardiova...

We have determined distinct roles for different proteasome complexes in adenovirus (Ad) E1A-dependent transcription. We show that the 19S ATPase, S8, as a component of 19S ATPase proteins independent of 20S (APIS), binds specifically to the E1A transactivation domain, conserved region 3 (CR3). Recruitment of APIS to CR3 enhances the ability of E1A to stimulate transcription from viral early gene promoters during Ad infection of human cells. The ability of CR3 to stimulate transcription in yeast is similarly dependent on the functional integrity of yeast APIS components, Sug1 and Sug2. The 20S proteasome is also recruited to CR3 independently of APIS and the 26S proteasome. Chromatin immunoprecipitation reveals that E1A, S8 and the 20S proteasome are recruited to both Ad early region gene promoters and early region gene sequences during Ad infection, suggesting their requirement in both transcriptional initiation and elongation. We also demonstrate that E1A CR3 transactivation and degradation sequences functionally overlap and that proteasome inhibitors repress E1A transcription. Taken together, these data demonstrate distinct roles for APIS and the 20S proteasome in E1A-dependent transactivation.

Unliganded thyroid hormone (TH) receptors (TRs) and other nuclear receptors (NRs) repress transcription of hormone-activated genes by recruiting corepressors (CoRs), such as NR CoR (N-CoR) and SMRT. Unliganded TRs also activate transcription of THrepressed genes. Some evidence suggests that these effects also involve TR͞CoR contacts; however, the precise reasons that CoRs activate transcription in these contexts are obscure. Unraveling these mechanisms is complicated by the fact that it is difficult to decipher direct vs. indirect effects of TR-coregulator contacts in mammalian cells. In this study, we used yeast, Saccharomyces cerevisiae, which lack endogenous NRs and NR coregulators, to determine how unliganded TRs can activate transcription. We previously showed that adenovirus 5 early-region 1A coactivates unliganded TRs in yeast, and that these effects are blocked by TH. We show here that human adenovirus type 5 early region 1A (