GATA1 and the BET Family Protein Brd3 Form a Mitotic Bookmarking Complex

Abstract 282

Erythroid-specific transcription patterns are maintained throughout cell division. During mitosis, transcription is silenced globally. This raises the question whether mechanisms are in place that ensure the spatially and temporally correct reassembly of transcriptional regulators and thus maintain lineage fidelity. We recently found that, in contrast to most nuclear regulators, the master hematopoietic regulator GATA1 remains associated at a subset of its targets within mitotic chromosomes in erythroid cells (Kadauke et al., Cell 2012). GATA1 appears to function by creating an epigenetic “bookmark” to facilitate timely post-mitotic transcription reactivation of its mitotic target genes.

GATA1 is acetylated at two lysine-rich domains near its zinc finger domains. We recently discovered that acetylated GATA1 recruits the double bromodomain protein Brd3 to erythroid target genes (Lamonica et al., PNAS 2011). Brd3 interacts with acetylated GATA1 via its first bromodomain, and Brd3 recruitment to GATA1 target sites is critically required for induction of terminal erythroid target genes such as α- and β-globin. Notably, Brd3 belongs to a family of proteins (called the BET family) of which two members (Brd2 and Brd4) are known to be retained on mitotic chromosomes.

We now find by immunofluorescence and live cell confocal imaging that Brd3 globally binds to mitotic chromosomes. ChIP-seq experiments demonstrate a high degree of co-localization of Brd3 and GATA1 genome-wide both in interphase and in mitosis. We further demonstrate that GATA1 directly recruits Brd3 to mitotic GATA1 target sites. Transient mitosis-specific disruption of the Brd3-GATA1 interaction using the small molecule BET bromodomain inhibitor JQ1 removed Brd3, but not GATA1, from mitotic binding sites and led to a profound delay in the reactivation of GATA1-bookmarked genes. This suggests that Brd3 is an integral component of GATA1's bookmarking function. In concert, these studies support a requirement of mitotic bookmarking by a GATA1/Brd3 complex for the propagation of lineage-specific transcription programs in dividing erythroid cells.