Posttranslational modifications play important roles in regulating protein structure and function. Histone deacetylase 6 (HDAC6) is a mostly cytoplasmic class II HDAC, which has a unique structure with two catalytic domains and a domain binding ubiquitin with high affinity. This enzyme was recently identified as a multisubstrate protein deacetylase that can act on acetylated histone tails, ␣-tubulin and Hsp90. To investigate the in vivo functions of HDAC6 and the relevance of tubulin acetylation/deacetylation, we targeted the HDAC6 gene by homologous recombination in embryonic stem cells and generated knockout mice. HDAC6-deficient mice are viable and fertile and show hyperacetylated tubulin in most tissues. The highest level of expression of HDAC6 is seen in the testis, yet development and function of this organ are normal in the absence of HDAC6. Likewise, lymphoid development is normal, but the immune response is moderately affected. Furthermore, the lack of HDAC6 results in a small increase in cancellous bone mineral density, indicating that this deacetylase plays a minor role in bone biology. HDAC6-deficient mouse embryonic fibroblasts show apparently normal microtubule organization and stability and also show increased Hsp90 acetylation correlating with impaired Hsp90 function. Collectively, these data demonstrate that mice survive well without HDAC6 and that tubulin hyperacetylation is not detrimental to normal mammalian development.Protein acetylation/deacetylation is involved in the regulation of protein structure and function, and therefore has potentially important roles in most cellular processes. In particular, the impact of histone N-terminal acetylation on chromatin organization and gene expression has been well documented (15). Acetylation and deacetylation of histone tails or of other proteins are catalyzed by histone acetyltransferases and histone deacetylases (HDACs), respectively. In mammals, there are 18 HDACs identified so far that can be grouped into three classes (reviewed in references 35, 36, and 39). In cells, most, if not all, class I and II HDACs are part of high-molecular-weight complexes that typically contain several HDAC polypeptides and are recruited to DNA via their interactions with sequence-specific or nonspecific DNA-binding proteins.HDAC 6 (HDAC6) was first identified through its homology to the Saccharomyces cerevisiae histone deacetylase HDA1 (9, 34). Like other class II HDACs, HDAC6 is mainly localized in the cytoplasm, but it can also shuttle between the nucleus and cytoplasm (33). This process is regulated by an N-terminally located nuclear export signal and possibly other uncharacterized mechanisms. HDAC6 has not been found in any class I or II HDAC-containing repressor complexes, which suggests it may have a unique regulation and possibly substrates different from those of other HDACs. However, it was shown biochemically and in genome-wide two-hybrid experiments to associate with the class III deacetylase SirT2 (22,26). Interestingly, HDAC6 contains two hdac catalyti...

Histone deacetylases (HDACs) regulate gene expression by deacetylating histones and also modulate the acetylation of a number of nonhistone proteins, thus impinging on various cellular processes. Here, we analyzed the major class I enzymes HDAC1 and HDAC2 in primary mouse fibroblasts and in the B-cell lineage. Fibroblasts lacking both enzymes fail to proliferate in culture and exhibit a strong cell cycle block in the G1 phase that is associated with up-regulation of the CDK inhibitors p21 WAF1/CIP1 and p57 Kip2 and of the corresponding mRNAs. This regulation is direct, as in wild-type cells HDAC1 and HDAC2 are bound to the promoter regions of the p21 and p57 genes. Furthermore, analysis of the transcriptome and of histone modifications in mutant cells demonstrated that HDAC1 and HDAC2 have only partly overlapping roles. Next, we eliminated HDAC1 and HDAC2 in the B cells of conditionally targeted mice. We found that B-cell development strictly requires the presence of at least one of these enzymes: When both enzymes are ablated, B-cell development is blocked at an early stage, and the rare remaining pre-B cells show a block in G1 accompanied by the induction of apoptosis. In contrast, elimination of HDAC1 and HDAC2 in mature resting B cells has no negative impact, unless these cells are induced to proliferate. These results indicate that HDAC1 and HDAC2, by normally repressing the expression of p21 and p57, regulate the G1-to-S-phase transition of the cell cycle.[Keywords: Histone deacetylases; cell cycle control; p21; p57; B-cell development] Supplemental material is available at http://www.genesdev.org.

SUMMARY The transcriptional activators Oct4, Sox2 and Nanog cooperate with a wide array of cofactors to orchestrate an embryonic stem (ES) cell-specific gene expression program that forms the molecular basis of pluripotency. Here we report using an unbiased in vitro transcription-biochemical complementation assay to discover a multi-subunit stem cell coactivator complex (SCC) that is selectively required for the synergistic activation of the Nanog gene by Oct4 and Sox2. Purification, identification and reconstitution of SCC revealed this coactivator to be the trimeric XPC-nucleotide excision repair complex. SCC interacts directly with Oct4 and Sox2 and is recruited to the Nanog and Oct4 promoters as well as a majority of genomic regions that are occupied by Oct4 and Sox2. Depletion of SCC/XPC compromised both pluripotency in ES cells and somatic cell reprogramming of fibroblasts to induced pluripotent stem (iPS) cells. This study identifies a transcriptional coactivator with diversified functions in maintaining ES cell pluripotency and safeguarding genome integrity.