Multivalent Scaffolding by XIST E-Repeats Tunes RNA Binding Protein Cooperativity and Lowers the Phase Separation Threshold

Four E-repeats of XIST E-Repeat RNA enable PTBP1 and ELAVL1 to phase separate under minimal in vitro conditions, lowering the phase separation threshold.

• Why it matters: Understanding how E-Repeat RNA recruits multiple RNA-binding proteins to seed condensates is key to explaining how XIST spreads across the X chromosome for dosage compensation.
• What they did: The authors defined in vitro interactions between E-Repeat RNA and PTBP1, ELAVL1, TDP43, and hnRNPU using bioinformatics, in silico analyses, and biochemical assays with minimal E-Repeat constructs.
• The result: All proteins bind single E-Repeat RNA with nanomolar affinity, and with four E-repeats PTBP1 and ELAVL1 form higher-order assemblies showing competition/co-occupancy and phase separation that recapitulates cellular behavior.