Evolutionary Conservation of Structural and Functional Coupling between the BRM AT-Hook and Bromodomain.

The BAF chromatin remodeling complex is critical for genome regulation. The central ATPase of BAF is either BRM or BRG1, both of which contain a C-terminal bromodomain, known to associate with acetylated lysines. We have recently demonstrated that in addition to acetyl-lysine binding, the BRG1/BRM bromodomain can associate with DNA through a lysine/arginine rich patch that is adjacent to the acetyl-lysine binding pocket. Flanking the bromodomain is an AT-hook separated by a short, proline-rich linker. We previously found that the AT-hook and bromodomain can associate with DNA in a multivalent manner. Here, we investigate the conservation of this composite module and find that the AT-hook, linker, and lysine/arginine rich bromodomain patch are ancient, conserved over ~1 billion years. We utilize extensive mutagenesis, NMR spectroscopy, and fluorescence anisotropy to dissect the contribution of each of these conserved elements in association of this module with DNA. Our results reveal a structural and functional coupling of the AT-hook and bromodomain mediated by the linker. The lysine/arginine rich patch on the bromodomain and the conserved elements of the AT-hook are critical for robust affinity for DNA, while the conserved elements of the linker are dispensable for overall DNA affinity but critical for maintaining the relative conformation of the AT-hook and bromodomain in binding to DNA. This supports that the coupled action of the AT-hook and bromodomain are important for BAF activity.