The condensation of HP1-α/Swi6 imparts nuclear stiffness

Liquid-liquid phase separation (LLPS) has emerged as a major driver of cellular organization. However, it remains unexplored whether the mechanical properties of LLPS domains are functionally important. The heterochromatin protein HP1-α (and the orthologous Swi6 in S. pombe) is capable of LLPS in vitro and promotes formation of LLPS heterochromatin domains in vivo. Here, we demonstrate that LLPS of Swi6 contributes to the emergent mechanical properties of nuclei. Using nuclear fluctuation analysis in live cells and force spectroscopy of isolated nuclei, we find that disrupting histone H3K9 methylation or depleting Swi6 compromises nuclear stiffness, while heterochromatin spreading through loss of the H3K9 demethylase, Epe1, increases nuclear stiffness. Leveraging a separation-of-function allele, we demonstrate that phase separation of Swi6—but not its histone binding or dimerization—is essential for Swi6’s mechanical role. These findings demonstrate that altering chromatin state has mechanical consequences and highlights that phase-separated domains can do mechanical work.