Heterochromatin, a type of condensed DNA in eukaryotic cells, has two main categories: Constitutive heterochromatin, which contains H3K9 methylation, and facultative heterochromatin, which contains H3K27 methylation. Methylated H3K9 and H3K27 serve as docking sites for chromodomain-containing proteins that compact chromatin. M33 (also known as CBX2) is a chromodomain-containing protein that binds H3K27me3 and compacts chromatin in vitro. However, whether M33 mediates chromatin compaction in cellulo remains unknown. Here we show that M33 compacts chromatin into DAPI-intense heterochromatin domains in cells. The formation of these heterochromatin domains requires H3K27me3, which recruits M33 to form nuclear bodies. G9a and SUV39H1 are sequentially recruited into M33 nuclear bodies to create H3K9 methylated chromatin in a process that is independent of HP1α. Finally, M33 decreases progerin-induced nuclear envelope disruption caused by loss of heterochromatin. Our findings demonstrate that M33 mediates the formation of condensed chromatin by forming nuclear bodies containing both H3K27me3 and H3K9me3. Our model of M33-dependent chromatin condensation suggests H3K27 methylation corroborates with H3K9 methylation during the formation of facultative heterochromatin and provides the theoretical basis for developing novel therapies to treat heterochromatin-related diseases.

异染色质是真核细胞中的一种浓缩 DNA，有两大类：组成型异染色质，含有 H3K9 甲基化，以及兼性异染色质，含有 H3K27 甲基化。甲基化的 H3K9 和 H3K27 充当压缩染色质的含染色质域蛋白质的对接位点。M33（也称为 CBX2）是一种含有染色质结构域的蛋白质，可在体外结合 H3K27me3 并压缩染色质。然而，M33 是否介导了纤维素中的染色质压实？仍然未知。在这里，我们展示了 M33 将染色质压缩成细胞中 DAPI 密集的异染色质域。这些异染色质结构域的形成需要 H3K27me3，它可以招募 M33 形成核体。G9a 和 SUV39H1 依次被募集到 M33 核体中，以在一个独立于 HP1α 的过程中产生 H3K9 甲基化染色质。最后，M33 减少了由异染色质丢失引起的早衰蛋白诱导的核膜破坏。我们的研究结果表明，M33 通过形成包含 H3K27me3 和 H3K9me3 的核体来介导凝聚染色质的形成。