Eukaryotic DNA is organised into chromatin, the architecture of which must be tightly regulated to ensure proper gene expression. Chromatin remodelling enzymes control the composition and density of chromatin, in part by repositioning the histone octamers that package DNA into nucleosomes. However, these enzymes do not display DNA sequence specificity, and it is currently not known how they localize to their cognate targets.
We have recently developed several biophysical assays to measure the nucleosome sliding activity of CHD4, an essential chromatin remodeler found in all complex organisms. Our data reveal that CHD4 activity is enhanced the transcriptional co-regulator BRD4. BRD4 is a reader of lysine acetylation that is localized to the promoters of active genes, which display elevated levels of histone acetylation.
To determine the structural basis for this rate enhancement, we have sought to assemble a ternary complex comprising CHD4, BRD4 and a nucleosome and determine its three-dimensional structure using cryoelectron microscopy. Initial screening of the complex using negative-stain electron microscopy produced micrographs showing intact particles of an appropriate size that did not appear to have problems with aggregation. Collection of preliminary cryoelectron microscopy data for the same complex has been used to produce a low-resolution structure of the complex, providing us with a strong starting point for sample optimisation. The data suggest that the determination of a high-resolution structure of the complex is feasible. Such a structure would enhance our understanding of how BRD4 may be involved in the regulation of the chromatin remodelling activity of CHD4.