High affinity binding of H3K14ac through collaboration of bromodomains 2, 4 and 5 is critical for the molecular and tumor suppressor functions of PBRM 1

Polybromo‐1 ( PBRM 1) is an important tumor suppressor in kidney cancer. It contains six tandem bromodomains ( BD s), which are specialized structures that recognize acetyl‐lysine residues. While BD 2 has been found to bind acetylated histone H3 lysine 14 (H3K14ac), it is not known whether other BD s collaborate with BD 2 to generate strong binding to H3K14ac, and the importance of H3K14ac recognition for the molecular and tumor suppressor function of PBRM 1 is also unknown. We discovered that full‐length PBRM 1, but not its individual BD s, strongly binds H3K14ac. BD s 2, 4, and 5 were found to collaborate to facilitate strong binding to H3K14ac. Quantitative measurement of the interactions between purified BD proteins and H3K14ac or nonacetylated peptides confirmed the tight and specific association of the former. Interestingly, while the structural integrity of BD 4 was found to be required for H3K14ac recognition, the conserved acetyl‐lysine binding site of BD 4 was not. Furthermore, simultaneous point mutations in BD s 2, 4, and 5 prevented recognition of H3K14ac, altered promoter binding and gene expression, and caused PBRM 1 to relocalize to the cytoplasm. In contrast, tumor‐derived point mutations in BD 2 alone lowered PBRM 1's affinity to H3K14ac and also disrupted promoter binding and gene expression without altering cellular localization. Finally, overexpression of PBRM 1 variants containing point mutations in BD s 2, 4, and 5 or BD 2 alone failed to suppress tumor growth in a xenograft model. Taken together, our study demonstrates that BD s 2, 4, and 5 of PBRM 1 collaborate to generate high affinity to H3K14ac and tether PBRM 1 to chromatin. Mutations in BD 2 alone weaken these interactions, and this is sufficient to abolish its molecular and tumor suppressor functions.