Characterisation of MiDAC, a class I Histone Deacetylase co-repressor Complex

The class I histone deacetylases (HDAC) are zinc dependent enzymes that, with the exception of HDAC8, require recruitment into multi-component co-repressor complexes to carry out their full enzymatic activity. Such co-repressor complexes are essential for life and are found in all organisms from yeast to humans. It is believed that class I HDAC containing co-repressor complexes predominantly act on lysine residues in histone tails to bring about chromatin compaction and gene silencing. The HDAC enzymes are upregulated in numerous cancers and have been implicated in neurodegenerative diseases. Given the importance of HDAC enzymes in growth, development and homeostasis and their potential usefulness as therapeutic targets, it is essential to gain a better understanding of their mechanism of action. A greater understanding of co-repressor complexes will allow for a clearer understanding of chromatin dynamics whilst also offering better tools for a more targeted approach in developing therapeutics. A novel class I HDAC containing co-repressor complex was recently discovered in a chemoproteomics screen. This complex was found in higher amounts in cells arrested in mitosis and subsequently termed the Mitotic Deacetylase Complex (MiDAC). In this thesis I have utilised structural and biophysical techniques to gain a greater understanding of the MiDAC complex. Initially work was performed to understand whether the MiDAC complex, like other known co-repressors such as SMRT and NuRD, was regulated by inositol phosphates. Indeed, it has been shown that inositol phosphates do regulate the HDAC activity of MiDAC. Binding assays have subsequently been used to determine the affinity of MiDAC for inositol phosphate and an allosteric mechanism of inositol phosphate regulation is proposed. Enzyme assays have demonstrated that MiDAC can select between histone tails. Structural techniques show that the MiDAC complex exists as a tetramer and a model of the MiDAC complex assembly has been proposed.