Question
Do K63-linked ubiquitin chains accumulate on G3BP1 during chronic stress to increase stress granule viscosity and block dissolution, and does deubiquitylation or competition with K63-chain binding re-liquify pathological granules and reduce TDP-43 cytoplasmic sequestration?
ALS/FTD-linked RNA-binding proteins including TDP-43 undergo liquid-to-solid phase transitions within stress granules that are pathologically relevant but mechanistically underexplored. G3BP1 is a core stress granule nucleator that regulates condensate material properties. K63-linked ubiquitylation of G3BP1 during chronic stress is proposed to increase intermolecular bridging and reduce condensate fluidity, effectively 'locking' stress granules and sequestering RBPs. Disrupting K63-chain formation or bridging offers a therapeutic strategy distinct from solubilizing aggregates already formed. The challenge requires: (1) mass-spec identification of G3BP1 ubiquitylation sites under chronic arsenite stress; (2) K63-specific deubiquitylase (AMSH-LP) treatment and fluorescence recovery after photobleaching to measure condensate fluidity; (3) TDP-43 nuclear vs. cytoplasmic distribution rescue by K63-chain competition; (4) iPSC-motor neuron validation. Falsifiable prediction: K63-chain competition (using K63-UIM decoy) should restore condensate fluidity (FRAP t1/2 ≤5× control) in ≥70% of cells and reduce cytoplasmic TDP-43 fraction by ≥40% in ALS iPSC-motor neurons under chronic stress. Bounty tier: $500K condensate biology / ALS-FTD mechanistic study.