Hbo1 and Msl complexes preserve differential compaction and H3K27me3 marking of active and inactive X chromosomes during mitosis

In mammals, chromosome-wide regulatory mechanisms ensure a balance of X-linked gene dosage between males (XY) and females (XX). In female cells, expression of genes from one of the two X-chromosomes is curtailed, with selective accumulation of Xist-RNA, Xist-associated proteins, specific histone modifications (eg. H3K27me3) and Barr body formation observed throughout interphase. Using chromosome flow-sorting, we show that during mitosis, Xist-associated proteins dissociate from inactive X (Xi) chromosomes, while high levels of H3K27me3 and increased compaction of the Xi relative to active X (Xa), are retained. Proteomic comparison of mitotic Xi and Xa revealed, unexpectedly, that components of Hbo1 and Msl/Mof histone acetyltransferase complexes co-enrich with Xa, while inhibitors of histone acetylation co-enrich with Xi. Furthermore, inhibition of Hbo1 or deletion of Msl/Mof components functionally abolishes mitotic differences in H3K27me3 marking and chromosome compaction. These data uncover critical roles for acetylation pathways in preserving X chromosome properties during mitosis.