Bromodomains regulate dynamic targeting of the PBAF chromatin remodeling complex to chromatin hubs

Abstract

Transcriptional bursting involves genes rapidly switching between active and inactive states. Chromatin remodelers actively target arrays of acetylated nucleosomes at select enhancers and promoters to facilitate or shut down the repeated recruitment of RNA Pol II during transcriptional bursting. It is unknown how acetylated chromatin is dynamically targeted and regulated by chromatin remodelers such as PBAF. Thus, we sought to understand how PBAF targets acetylated chromatin using live-cell single molecule fluorescence microscopy. Our work reveals chromatin hubs throughout the nucleus where PBAF rapidly cycles on and off the genome. Deletion of PBAFs bromodomains impairs targeting, stable engagement and persistent binding on chromatin in hubs. Interestingly, PBAF has a higher probability to stably engage chromatin inside hubs indicating that hubs contain a unique nucleosomal scaffold compared to global chromatin. Dual color imaging of PBAF in hubs near H3.3 or HP1 reveals that PBAF targets both euchromatic and heterochromatic regions with distinct genome binding kinetics that mimic chromatin stability. Removal of PBAFs bromodomains stabilizes H3.3 and HP1 binding within chromatin indicating that bromodomains may play a direct role in remodeling of the nucleosome. Our data, suggests that PBAF differentially and dynamically engages a variety of chromatin structures involved in both activation and repression of transcription via bromodomains. Furthermore, PBAFs binding stability on chromatin may reflect the chromatin remodeling potential of different bound chromatin states. Statement of SignificanceTranscriptional bursting involves a gene rapidly switching between transcriptionally active and inactive states. To regulate transcriptional bursting, chromatin must interchange between euchromatin and heterochromatin to permit or restrict access of transcription factors including RNA Polymerase II to enhancer and gene promoters. However, little is known regarding how chromatin remodelers dynamically read a rapidly changing 4D epigenome. We used live-cell single molecule imaging to characterize the spatiotemporal chromatin binding dynamics of PBAF, a chromatin remodeler that accesses both euchromatin and heterochromatin to regulate transcription. PBAF cycles on and off chromatin hubs in select nuclear regions where it distinctly engages euchromatin and heterochromatin via bromodomains in its BAF180 subunit. Our study provides the framework to understand how the 4D epigenome is regulated.