BRD4 Phosphorylation Regulates the Structure of Chromatin Nanodomains.

The interplay between chromatin structure and phase-separating proteins is an emerging topic in cell biology with implications for understanding disease states. Here, we investigate the functional relationship between bromodomain protein 4 (BRD4) and chromatin architecture. By combining molecular dynamics simulations with live-cell imaging, we demonstrate that BRD4, when mutated at specific N-terminus sites, significantly impacts the organization and dynamics of chromatin nanodomains, known as nucleosome clutches. Our findings reveal that a constitutively phosphorylated mutant of BRD4 condenses nucleosome clutches, while treatment with (+)-JQ1 increases the diffusion dynamics of single nucleosomes and decondenses nucleosome clutches. Simultaneously, we demonstrate that BRD4 mutations can alter localization of BRD4 to chromatin as well as modify single nucleosome dynamics. These results suggest that both chromatin binding and phase separation of BRD4 could co-regulate the nanoscale chromatin architecture and the chromatin microenvironment. Our observations shed light on the nuanced regulation of chromatin structure by BRD4, offering insights into its role in maintaining the nuclear architecture and transcriptional activity.