Multivalent Targeting of Nucleosomes by the BRG1 At-Hook and Bromodmain

Abstract

Beyond providing an avenue for packaging the genome into the nucleus, chromatin provides elegant mechanisms for the dynamic regulation of the eukaryotic genome. Chromatin structure undergoes dramatic spatial and temporal reorganizations throughout the life cycle of the cell, which require extensive remodeling of the nucleosome, the basic subunit of chromatin. A major mechanism for modulating chromatin structure is through ATP driven nucleosome remodeling. The switching/sucrose non-fermenting (SWI/SNF) chromatin remodeling complex facilitates ATP dependent remodeling of nucleosomes critical in gene regulation. SWI/SNF activity is mediated by one of two possible ATPases, Brahma (BRM) or Brahma related gene 1 (BRG1). The chromatin targeting, occupancy and activity of the complex are positively affected by histone acetylation, and it has been found that this is mediated through the C-terminal Bromodomains (BDs) of BRG1 and BRM. One possible mechanism for this is that BD recognition of acetylated histones targets and/or retains SWI/SNF at chromatin leading to increased activity. This is supported by previous reports of acetylation and BD dependent recruitment and retention of SWI/SNF at gene promoters. However, the BRG1 BD has been shown to have very poor affinity and conflicting specificity for acetylated histone peptides in vitro, calling into question the importance of this interaction. We propose that the nucleosome context is critical for proper activity of the BD in binding acetylated histones, and suggest that an adjacent AT-hook DNA binding domain contributes to BD activity. Here we present our recent results using NMR spectroscopy and TIRFM to investigate the multivalent interaction of the BRG1 AT-hook and BD with nucleosomes, including details of the structural basis by which the AT-hook-BD motif interacts with the 601-mononucleosome, the kinetic and thermodynamic basis of complex formation, as well as the effect of histone acetylation.