Nuclear-import receptors remodel the dilute phase to suppress phase transitions of RNA-binding proteins with prion-like domains.

AbstractRNA-binding proteins (RBPs) with prion-like domains, including FUS, hnRNPA1, and hnRNPA2, assemble into functional, metastable condensates that organize ribostasis, but can also transition into self-templating fibrils implicated in neurodegenerative proteinopathies such as amyotrophic lateral sclerosis (ALS). How nuclear-import receptors (NIRs) antagonize this pathological transition has remained unresolved. Here, we establish that NIRs regulate the phase behavior of prion-like cargos by remodeling the dilute phase. Quantitative analyses across length scales reveal that Karyopherin-{beta}2 (Kap{beta}2) preferentially binds cargo in the dilute phase to lower the effective concentration of free RBPs thereby elevating the saturation concentration for phase separation and suppressing mesoscale clustering. ALS-linked FUSP525L, which binds Kap{beta}2 weakly, evades this regulation to form pathogenic assemblies. Thus, NIRs harness polyphasic linkage, the thermodynamic relationship between ligand binding and phase equilibria, to reshape the landscape of prion-like RBP assembly states, establishing a paradigm for how ATP-independent chaperones regulate phase behavior to prevent disease-linked aggregation. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=123 SRC="FIGDIR/small/688546v1_ufig1.gif" ALT="Figure 1"> View larger version (31K): org.highwire.dtl.DTLVardef@1dd32cforg.highwire.dtl.DTLVardef@132ba1eorg.highwire.dtl.DTLVardef@1593c80org.highwire.dtl.DTLVardef@19311d6_HPS_FORMAT_FIGEXP M_FIG C_FIG