Mocavini, I. | Search bioRxiv

bioRxiv2026

Functional interplay between H2B ubiquitylation and H2A.Z deposition

The Bre1 ubiquitin conjugating enzyme catalyzes the monoubiquitylation of histone H2B-K120/K123 at promoter proximal nucleosomes, contributing to transcriptional regulation. These same nucleosomes are targeted by the yeast SWR1C chromatin remodeling enzyme that deposits the histone variant H2A.Z. Yeast strains that lack both Bre1 and Swr1 are inviable, indicating that together they contribute to an essential cell function. Interestingly, H2B-K123ub and H2A.Z levels are anticorrelated, and recent biochemical studies suggest a model in which H2B-K123ub inhibits SWR1C activity by blocking access to the nucleosome acidic patch. Here, we exploit recombinant nucleosomes to show that the H2A.Z deposition activity of SWR1C is strongly inhibited by H2B-K123ub. Surprisingly, the loss of H2B-K123ub does not lead to significant changes in the genomic organization of H2A.Z in cells grown in normal media, but we find that H2B-K123ub is required for a re-localization of H2A.Z from promoters to coding regions during replication stress. Together, these data indicate a complex functional interplay between H2B ubiquitylation and H2A.Z deposition.

biochemistry↗

bioRxiv2023

Alternative splicing decouples local from global PRC2 activity

The Polycomb repressive complex 2 (PRC2) mediates epigenetic maintenance of gene silencing in eukaryotes via methylation of histone H3 at lysine 27 (H3K27). This complex interacts with sets of accessory factors to form two distinct subtypes, PRC2.1 and PRC2.2, that differ in their actions and chromatin targeting mechanisms. The underlying molecular mechanisms orchestrating PRC2 assembly are not yet fully understood. Here, we report that alternative splicing (AS) of the PRC2 core component SUZ12 generates an uncharacterised isoform SUZ12-S, which co-exists with the canonical SUZ12-L isoform in the same cell in virtually all tissues and developmental stages. We show that SUZ12-S drives PRC2.1 subtype formation and favours PRC2 dimerisation. While SUZ12-S is necessary and sufficient to ensure correct repression of Polycomb target genes via promoter-proximal H3K27me3 deposition, SUZ12-L is needed for maintaining global H3K27 methylation levels. Lastly, we show that mouse embryonic stem cells (ESCs) lacking either of the two isoforms have a slower exit from pluripotency upon differentiation stimulus. Our findings thus reveal a physiological mechanism regulating PRC2 assembly and higher-order interactions, with impacts on H3K27 methylation and gene repression.

molecular biology↗