HDAC8 can catalyze the in vitro deacetylation of a number of acetylated histone variants including full-length H2A/H2B, H3, and H4 histones acetylated at nonspecific lysines (Hu et al. 2000, Buggy et al. 2000). Peptide sequences corresponding to the H4 histone tail with an acetylated lysine at position sixteen (AcK16) were also identified as in vitro substrates (Buggy et al. 2000, Van der Wyngaert et al. 2000). Subsequent studies have used the H4 histone tail sequence as a peptide template to investigate the amino acid sequence preference of HDAC8. HDAC8 can catalyze the in vitro deacetylation of AcK20 on the H4 histone tail though at a much slower rate than deacetylation of AcK16 peptides (Dose et al. 2011). HDAC8 can catalyze deacetylation in vivo in the absence of a protein complex (Dowling et al. 2010). The role of HDAC8 in catalyzing deacetylation of specific sites in histones in vivo remains unclear (Wolfson et al. 2013).

Histone deacetylase HDAC8 deacetylates SMC3 cohesin subunit. SMC3 deacetylation promotes dissociation of cleaved RAD21 fragments from other cohesin proteins and their replacement with intact RAD21, thereby allowing restoration of the cohesin complex (Deardorff et al. 2012). HDAC8 mutations, as well as mutations in NIPBL, SMC1A and SMC3, can cause Cornelia de Lang syndrome (Deardorff et al. 2012).

HDAC8 deacetylates cohesin in prometaphase, after cohesin dissociates from chromosomal arms (Deardorff et al. 2012).

ESPL1 (separin i.e. separase) cleaves RAD21 (SCC1) subunit of centromeric cohesin at two sites that conform to the consensus separase recognition site E-X-X-R: after arginine residue R172 and after arginine residue R450 (Hauf et al. 2001). Phosphorylation of RAD21 at the serine residue S454 by PLK1 in prometaphase facilitates ESPL1-mediated cleavage of RAD21 at the C-terminal cleavage site R450 (Hauf et al. 2005). The N-terminal and C-terminal RAD21 cleavage fragments remain bound to the rest of the cohesin complex (Deardorff et al. 2012). It is not clear whether RAD21 middle fragment also continues to be associated with cohesin.

Acetyltransferases ESCO1 and ESCO2 are homologs of the S. cerevisiae acetyltransferase Eco1, essential for viability in yeast. ESCO1 and ESCO2 share sequence homology in the C-terminal region, consisting of a H2C2 zinc finger motif and an acetyltransferase domain (Hou and Zou 2005). Both ESCO1 and ESCO2 acetylate the cohesin subunit SMC3 on two lysine residues, K105 and K106 (Zhang et al. 2008), an important step in the establishment of sister-chromatid cohesion during the S-phase of the cell cycle. These dual acetylations on SMC3 are deacetylated by HDAC8 after the cohesin removal from chromatin for the dissociation and recycling of cohesin subunits (Deardorff et al. 2012). ESCO1 and ESCO2 differ in their N-termini, which are necessary for chromatin binding, and may perform distinct functions in sister chromatid cohesion (Hou and Zou 2005), as suggested by the study of Esco2 knockout mice (Whelan et al. 2012).

Lysine deacetylases (KDACs), historically referred to as histone deacetylases (HDACs), are divided into the Rpd3/Hda1 metal-dependent 'classical HDAC family' (de Ruijter et al. 2003, Verdin et al. 2003) and the unrelated sirtuins (Milne & Denu 2008). Phylogenetic analysis divides human KDACs into four classes (Gregoretti et al. 2004): Class I includes HDAC1, 2, 3 and 8; Class IIa includes HDAC4, 5, 7 and 9; Class IIb includes HDAC6 and 10; Class III are the sirtuins (SIRT1-7); Class IV has one member, HDAC11 (Gao et al. 2002). Class III enzymes use an NAD+ cofactor to perform deacetylation (Milne & Denu 2008, Yang & Seto 2008), the others classes use a metal-dependent mechanism (Gregoretti et al. 2004) to catalyze the hydrolysis of acetyl-L-lysine side chains in histone and non-histone proteins yielding L-lysine and acetate. X-ray crystal structures are available for four human HDACs; these structures have conserved active site residues, suggesting a common catalytic mechanism (Lombardi et al. 2011). They require a single transition metal ion and are typically studied in vitro as Zn2+-containing enzymes, though in vivo HDAC8 exhibits increased activity when substituted with Fe2+ (Gantt et al. 2006). The structurally-related enzyme acetylpolyamine amidohydrolase (APAH) (Leipe & Landsman 1997) exhibits optimal activity with Mn2+, followed closely by Zn2+ (Sakurada et al. 1996).

HDACs are often part of multi-protein transcriptional complexes that are recruited to gene promoters, regulating transcription without direct DNA binding. With the exception of HDAC8, all class I members can be catalytic subunits of multiprotein complexes (Yang & Seto 2008). HDAC1 and HDAC2 interact to form the catalytic core of several multisubunit complexes including Sin3, nucleosome remodeling deacetylase (NuRD) and corepressor of REST (CoREST) complexes (Grozinger & Schreiber 2002). HDAC3 is part of the silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) complex or the homologous nuclear receptor corepressor (NCoR) (Li et al. 2000, Wen et al. 2000, Zhang et al. 2002, Yoon et al. 2003, Oberoi et al. 2011) which are involved in a wide range of processes including metabolism, inflammation, and circadian rhythms (Mottis et al. 2013).

Class IIa HDACs (HDAC4, -5, -7, and -9) shuttle between the nucleus and cytoplasm (Yang & Seto 2008, Haberland et al. 2009). The nuclear export of class IIa HDACs requires phosphorylation stimulated by calcium or other stimuli. They appear to have been evolutionarily inactivated as enzymes, having acquired a histidine substitution of the tyrosine residue in the active site of the mammalian deacetylase domain (H976 in humans) (Lahm et al. 2007, Schuetz et al. 2008). Instead they function as transcriptional corepressors for the MEF2 family of transcription factors (Yang & Gregoire 2005) .

Histones are the primary substrate for most HDACs except HDAC6 which is predominantly cytoplasmic and acts on alpha-tublin (Hubbert et al. 2002, Zhang et al. 2003, Boyault et al. 2007). HDACs also deacetylate proteins such as p53, E2F1, RelA, YY1, TFIIE, BCL6 and TFIIF (Glozak et al. 2005).

Histone deacetylases are targeted by structurally diverse compounds known as HDAC inhibitors (HDIs) (Marks et al. 2000). These can induce cytodifferentiation, cell cycle arrest and apoptosis of transformed cells (Marks et al. 2000, Bolden et al. 2006). Some HDIs have significant antitumor activity (Marks and Breslow 2007, Ma et al. 2009) and at least two are approved anti-cancer drugs.

The coordinates of post-translational modifications represented and described here follow UniProt standard practice whereby coordinates refer to the translated protein before any further processing. Histone literature typically refers to coordinates of the protein after the initiating methionine has been removed. Therefore the coordinates of post-translated residues in the Reactome database and described here are frequently +1 when compared with the literature.