HP1 dimerization drives the condensation and segregation of heterochromatin
Heterochromatin protein 1 (HP1) is a conserved chromatin-associated factor implicated in the condensation of H3K9me-marked heterochromatin, potentially through phase separation. However, whether HP1-mediated chromatin condensation primarily depends on dimerization or liquid–liquid phase separation (LLPS) remains unresolved. Using the C. elegans HP1 orthologue HPL-2 and a combined in vitro–in vivo approach, we systematically dissected the molecular determinants of HPL-2 function in heterochromatin condensation. Through functional mutants, we demonstrate that HPL-2 dimerization, but not LLPS, is essential for condensing H3K9me3 chromatin arrays in vitro and for maintaining canonical heterochromatin foci in C. elegans embryos. LLPS plays only an auxiliary role, enhancing or stabilising condensation rather than driving it. Moreover, HPL-2 dimerization is sufficient to mediate segregation of H3K9me3 from H3K9me0 chromatin arrays in vitro, generating biphasic condensates reminiscent of heterochromatin-like domains. Surprisingly, transcriptional analysis revealed that heterochromatin condensation and gene silencing are not tightly coupled: loss of condensation produced only minor transcriptional changes at canonical heterochromatin loci. Nonetheless, HPL-2 mutants defective in condensation caused profound physiological and developmental defects in C. elegans. Together, these findings establish dimerization as the principal mechanism of HP1-driven condensation, clarify the auxiliary role of LLPS, and uncover the uncoupling between heterochromatin condensation and transcriptional regulation.