ABSTRACT The ATPase family AAA+ domain containing 2 (ATAD2) protein, and its paralog ATAD2B, have a C-terminal bromodomain that functions as a ‘reader’ of acetylated lysine residues on histone proteins. Using a structure-function approach, we investigated the ability of the ATAD2 and ATAD2B bromodomains to select acetylated lysine among multiple histone post-translational modifications. Isothermal titration calorimetry experiments revealed that the ATAD2 and ATAD2B bromodomains selectively recognize distinct patterns of acetylated lysine residues on the N-terminal tails of histone proteins. Adjacent methylation or phosphorylation marks were found to either enhance or weaken the recognition of acetylated lysine by the ATAD2/B bromodomains. Complementary structural studies provide mechanistic insights into how residues within the bromodomain binding pocket coordinate the acetyllysine group in the context of adjacent post- translational modifications. Furthermore, we investigated how sequence changes in amino acids of the histone ligands, either as ‘onco’ mutations or as histone variants, impact the recognition of an adjacent acetylated lysine residue. In summary, our study highlights how the interplay between multiple combinations of histone modifications influences the ‘reader’ activity of the ATAD2 and ATAD2B bromodomains, resulting in distinct binding modes of the two bromodomains. KEY POINTS Multiple independent ATAD2 gene duplication events are evident during metazoan evolution, indicating expansion of functionality in the ATAD2 gene family and suggesting distinct functions for ATAD2 and ATAD2B. High-resolution structures of the ATAD2 and ATAD2B bromodomains in complex with their histone ligands demonstrate how multiple post-translational modifications are coordinated. Recognition of different subsets acetylated histone ligands by the ATAD2 and ATAD2B bromodomains is driven by unique features within the binding pockets of these paralogous proteins. Onco-histone mutations and histone variants that change the amino acid sequence of the histone tails modulate the ATAD2 and ATAD2B bromodomain activity. This study demonstrates how the combinatorial activity of multiple post- translational modifications forms a histone code and influences the recognition of acetylated lysine by bromodomain-containing proteins.

Impact of combinatorial histone modifications on acetyllysine recognition by the ATAD2 and ATAD2B bromodomains