NuMA promotes constitutive heterochromatin compaction by stabilizing linker histone H1 on chromatin

Heterochromatin exerts pivotal functions of silencing specific genes and maintenance of genome stability. However, its formation and maintenance mechanisms remain unclear. Here, we discover that the mitotic regulator NuMA, as a nucleoskeleton protein, is required for constitutive heterochromatin organization at the nucleosome level in interphase. NuMA depletion results in shortened nucleosome repeat length, dispersed nucleosome clutches, increased chromatin accessibility, and disrupted transcription repression of long terminal repeats in heterochromatin regions. Such functions of NuMA rely on its interaction with linker histone H1, which stabilizes H1's binding to chromatin and facilitates nucleosome stacking, as directly visualized by in situ cryo-ET. Notably, NuMA oligomerizes into quasi-meshwork in the nucleoplasm, providing its organization basis as a nucleoskeleton protein. Collectively, our findings illuminate the concerted effect of nucleoskeleton and linker histone on chromatin compaction at the nucleosome level, unveiling a previously unexplored mechanism by which nucleoskeleton regulates heterochromatin formation and maintenance.