ALS-linked mutations impair UBQLN2 stress-induced biomolecular condensate assembly in cells

Mutations in ubiquilin-2 (UBQLN2), a ubiquitin-binding shuttle protein involved in several protein quality control processes, can lead to amyotrophic lateral sclerosis (ALS). We previously found that wild-type UBQLN2 forms dynamic, membraneless biomolecular condensates upon cellular stress, and undergoes liquid-liquid phase separation in vitro. However, the impact of ALS-linked mutations on UBQLN2 condensate formation in cells remains unknown. Here, we overexpress mCherry-fused UBQLN2 with five patient-derived ALS-linked mutations and employ live-cell imaging and photokinetic analysis to investigate how each of these mutations impact stress-induced UBQLN2 condensate assembly and condensate material properties. Unlike endogenous UBQLN2, exogenously introduced UBQLN2 forms condensates distinct from stress granules. Both wild-type and mutant UBQLN2 condensates are generally cytoplasmic and liquid-like. However, mutant UBQLN2 forms fewer stress-induced UBQLN2 condensates than wild-type UBQLN2. Exogenously expressed P506T UBQLN2 forms the lowest number of stress-induced condensates of all UBQLN2 mutants, and these condensates are significantly smaller than those of wild-type UBQLN2. Fluorescence recovery after photobleaching (FRAP) analysis of UBQLN2 condensates revealed higher immobile fractions for UBQLN2 mutants, especially P506T. P497S and P497H mutations differentially impact condensate properties, demonstrating that the effects of ALS-linked mutations are both position- and amino acid-dependent. Collectively, our data show that disease mutations hinder assembly and alter viscoelastic properties of stress-induced UBQLN2 condensates, potentially leading to aggregates commonly observed in ALS.