Mi-2/NuRD complex

In the field of molecular biology, the Mi-2/NuRD (Nucleosome Remodeling Deacetylase) complex, is a group of associated proteins with both ATP-dependent chromatin remodeling and histone deacetylase activities.^[1][2] As of 2007^[update], Mi-2/NuRD was the only known protein complex that couples chromatin remodeling ATPase and chromatin deacetylation enzymatic functions.^[3]

In 1998, several independent groups reported the discovery of multi-enzyme complexes conferring both nucleosome remodelling and histone deacetylation activities.^[4][5][6][7] Xue et al^[1] first described the human complex as the Nucleosome Remodelling and Deacetylase (NuRD) - this name has since been adopted for homologous complexes in most organisms.

The NuRD complex contains seven subunits: the histone deacetylase core proteins HDAC1 and HDAC2, the histone-binding proteins RbAp46 and RbAp48, the metastasis-associated proteins MTA1 (or MTA2 / MTA3), the methyl-CpG-binding domain protein MBD3 (or MBD2) and the chromodomain-helicase-DNA-binding protein CHD3 (aka Mi-2alpha) or CHD4 (aka Mi-2beta).

NuRD can be subdivided into two discrete subcomplexes which confer neuclosome remodelling or histone deacetylation activity,each of which retains catalytic activity without the presence of the other.^[8] The histone deacetylases HDAC1 and HDAC2 and the histone binding proteins RbAp48 and RbAp46 form a core complex shared between NuRD and Sin3-histone deacetylase complexes.^[9][10]

Mi-2/CHD4 may confer NuRD independent transcriptional regulation in some organisms and contexts.^[11] For example, in the fly, Drosophila melanogaster, the majority of Mi2 biochemically purifies separately from the rest of the NuRD subunits^[12] and profiling of NuRD component binding sites indicates that only a minority of loci are co-occupied by both Mi-2 and HDAC.^[13] Similar results are reported in mouse embryonic stem cells where CHD4 shares only a minority of binding loci with core NuRD component, MBD3.^[14] Independently of histone deacetylase, Mi-2 knockdown in neuronal tissue results in mis-expression of genes that are normally restricted to germline.^[13] A similar observation was made in human erythroid cells, in which CHD4 but not Mi-2 is required for suppression of fetal globin genes.^[15]

NuRD is traditionally thought of as a primarily repressive complex (in AP-1^[16]), and in some contexts it is clear that it does confer this function. For example, NuRD is required to silence genes in neuronal differentiation.^[17] However, more recent studies have presented a more nuanced picture of NuRD activity in which it is required for fine-tuning of gene expression during stem cell differentiation to ensure appropriate lineage specification.^{[14][dubious – discuss]}