No abstract

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...