Cooperative IDR-folded domain interplay governs HP1α phase separation and underlies biphasic regulation by pathogenic poly-PR

HP1α undergoes liquid-liquid phase separation (LLPS), yet the prevailing model, which attributes condensation almost entirely to intrinsically disordered regions (IDRs), leaves the role of its folded domains unresolved. Using solution and solid-state NMR, we show that HP1α LLPS is instead governed by reciprocal interplay between IDRs and folded domains. Within the condensed phase, solid-state NMR reveals a dynamic hierarchy: the chromoshadow domain (CSD) assembles into polymorphic oligomers that form rigid cores, while the phosphorylation sites on the N-terminal extension (NTE) remain highly mobile, challenging their proposed role as stable multivalent hubs. Phosphorylation of the NTE instead promotes LLPS primarily by increasing local rigidity and releasing the NTE from the chromodomain (CD) to relieve autoinhibition, with electrostatic charge contributing secondarily. The C-terminal extension (CTE) suppresses phase separation through two routes: weakening the association between the hinge region (HR) and the CSD, and inhibiting interactions between CSD dimers. ALS-associated poly-PR targets this network at the NTE and exerts concentration-dependent biphasic regulation, promoting LLPS at low concentrations via multivalent bridging and dissolving condensates at high concentrations via saturation, a switch recapitulated in cells and in mouse brain. These findings reframe HP1α condensation from an IDR-driven process to one cooperatively governed by IDR-folded domain interplay.