Biology subjects

Agius, S. C.

Publications and source records attributed to Agius, S. C..

3 recordsLinked to original sources

bioRxiv2025

Accessory subunits of PRC2 mimic H3K27me3 to restrict the spread of Polycomb domains

The Polycomb repressive complex 2 (PRC2) is essential for normal development by maintaining developmental gene repression. PRC2 deposits the repressive chromatin mark H3 lysine 27 tri-methyl (H3K27me3) through a read-write loop that involves direct interactions between PRC2 and H3K27me3. According to current models, the PRC2-H3K27me3 read-write loop is initiated by the PRC2 subunits JARID2 and PALI1 that mimic H3K27me3. However, it is unknown what restricts the PRC2-H3K27me3 read-write loop from spreading H3K27me3 indefinitely. To answer this question, we generated mutant mice where PRC2 subunits cannot mimic H3K27me3. Unexpectedly, the mutations led to delayed Hox genes activation and a homeotic transformation characteristic of a Polycomb gain-of-function in vivo and the spread of H3K27me3 beyond Polycomb domains in stem cells. Collectively, we show that H3K27me3 mimicry evolved to compete against the PRC2-H3K27me3 read-write loop in a process that restrains PRC2 and restricts the spread of Polycomb domains. HighlightsO_LIH3K27me3 mimicry antagonises Polycomb function in vivo. C_LIO_LIJARID2 and PALI1 synergise to allosterically regulate PRC2 during development. C_LIO_LIH3K27me3 mimicry by JARID2 and PALI1 antagonises PRC2 in stem cells. C_LIO_LIJARID2 and PALI1 mimic H3K27me3 to restrict the spread of Polycomb domains. C_LI

molecular biology↗

bioRxiv2023

The apparent loss of PRC2 chromatin occupancy as an artefact of RNA depletion

RNA has been implicated in the recruitment of chromatin modifiers, and previous studies have provided evidence in favour and against this idea. RNase treatment of chromatin is a prevalent tool for the study of RNA-mediated regulation of chromatin modifiers, but the limitations of this approach remain unclear. RNase A treatment during chromatin immunoprecipitation (RNase-ChIP or rChIP) reduces chromatin occupancy of the H3K27me3 methyltransferase PRC2. This led to suggestions of an "RNA bridge" between PRC2 and chromatin. Here we show that RNase A treatment during chromatin immunoprecipitation leads to the apparent loss of all facultative heterochromatin, including both PRC2 and H3K27me3 genome wide. This phenomenon persists in mouse embryonic stem cells, human cancer cells and human-induced pluripotent stem cells. We track this observation to a gain of DNA from non-targeted chromatin, sequenced at the expense of DNA from facultative heterochromatin, which reduces ChIP signals. Our results point to substantial limitations in using RNase A treatment for mapping RNA-dependent chromatin occupancy and invalidate conclusions that were previously established for PRC2 based on this assay. HighlightsO_LIRNA degradation during ChIP-seq is insufficient to displace PRC2 from chromatin. C_LIO_LIRNA degradation led to the artificial depletion of ChIP-seq signals in multiple cell lines. C_LIO_LIArtificially reduced ChIP-seq signals are explained by a gain of non-targeted DNA. C_LIO_LIRNA is critical in maintaining the solubility of chromatin during experimentation. C_LI

molecular biology↗

bioRxiv2020

Convergent evolution between PALI1 and JARID2 for the allosteric activation of PRC2

The polycomb repressive complex 2 (PRC2) is a histone methyltransferase that maintains cell identities. JARID2 is the only accessory subunit of PRC2 that known to trigger an allosteric activation of methyltransferase. Yet, this mechanism cannot be generalised to all PRC2 variants as, in vertebrates, JARID2 is mutually exclusive with most of the accessory subunits of PRC2. Here we provide functional and structural evidence that the vertebrate-specific PRC2 accessory subunit PALI1 emerged through a convergent evolution to mimic JARID2 at the molecular level. Mechanistically, PRC2 methylates PALI1 K1241, which then binds to the PRC2-regulatory subunit EED to allosterically activate PRC2. PALI1 K1241 is methylated in mouse and human cell lines and is essential for PALI1-induced allosteric activation of PRC2. High-resolution crystal structures revealed that PALI1 mimics the regulatory interactions formed between JARID2 and EED. Independently, PALI1 also facilitates DNA and nucleosome binding by PRC2. In acute myelogenous leukemia cells, overexpression of PALI1 leads to cell differentiation, with the phenotype abrogated by a separation-of-function PALI1 mutation, defective in allosteric activation and active in DNA binding. Collectively, we show that PALI1 facilitates catalysis and substrate binding by PRC2 and provide evidence that subunit-induced allosteric activation is a general property of holo-PRC2 complexes.

biochemistry↗