PA28 (also known as PSME3), a nuclear activator of the 20S proteasome, is involved in the degradation of several proteins regulating cell growth and proliferation and in the dynamics of various nuclear bodies, but its precise cellular functions remain unclear. Here, using a quantitative FLIM-FRET based microscopy assay monitoring close proximity between nucleosomes in living human cells, we show that PA28 controls chromatin compaction. We find that its depletion induces a decompaction of pericentromeric heterochromatin, which is similar to what is observed upon the knockdown of HP1β (also known as CBX1), a key factor of the heterochromatin structure. We show that PA28 is present at HP1β-containing repetitive DNA sequences abundant in heterochromatin and, importantly, that HP1β on its own is unable to drive chromatin compaction without the presence of PA28. At the molecular level, we show that this novel function of PA28 is independent of its stable interaction with the 20S proteasome, and most likely depends on its ability to maintain appropriate levels of H3K9me3 and H4K20me3, histone modifications that are involved in heterochromatin formation. Overall, our results implicate PA28 as a key factor involved in the regulation of the higher order structure of chromatin.

PA28（也称为PSME3），是20S蛋白酶体的核激活剂，参与了几种调节细胞生长和增殖的蛋白质的降解以及各种核体的动力学，但其确切的细胞功能尚不清楚。在这里，使用基于FLIM-FRET的定量显微镜检测法，监测活人细胞中核小体之间的紧密接近性，我们显示PA28控制染色质的紧实。我们发现它的耗竭诱导了着丝粒体异染色质的分解，这类似于在敲除HP1β（也称为CBX1）（异染色质结构的关键因素）时观察到的现象。我们显示PA28存在于异染色质中含量丰富的HP1β重复DNA序列中，重要的是，如果没有PA28，HP1β本身无法驱动染色质紧实。在分子水平上，我们表明PA28的这一新功能独立于其与20S蛋白酶体的稳定相互作用，并且最有可能取决于其维持适当水平的H3K9me3和H4K20me3（参与异染色质形成的组蛋白修饰）的能力。总的来说，我们的研究结果暗示PA28是参与染色质高阶结构调控的关键因素。