Zn-dependent structural transition of SOD1 modulates its ability to undergo liquid-liquid phase separation.

The toxic gain of function of Cu/Zn superoxide dismutase (SOD1) associated with the neurodegenerative disease - Amyotrophic lateral sclerosis (ALS), is believed to occur via misfolding and/or aggregation. SOD1 is also associated with stress granules (SGs) which are a type of membraneless organelle believed to form via liquid-liquid phase separation (LLPS) of several proteins containing low-complexity, disordered regions. Using a combination of experiments and computer simulations, we report here that structural disorder in two loop regions of SOD1 induced by the absence of metal cofactor - Zn, triggers its LLPS. The phase-separated droplets give rise to aggregates which eventually form toxic amyloids upon prolonged incubation. The addition of exogenous Zn to immature, metal-free SOD1 and the severe ALS mutant - I113T, stabilized the loops and restored the folded structure, thereby inhibiting LLPS and subsequent aggregation. In contrast, the Zn-induced inhibition of LLPS and aggregation was found to be partial in the case of another severe ALS-associated mutant - G85R, which exhibits reduced Zn-binding. Moreover, a less-severe ALS mutant - G37R with perturbed Cu binding does not undergo LLPS. In conclusion, our work establishes a role for Zn-dependent modulation of SOD1 disorder and LLPS as a precursor phenomenon which may lead to the formation of toxic amyloids associated with ALS. Significance StatementThe formation of membraneless organelles such as stress granules (SGs) is believed to occur through the process of liquid-liquid phase separation (LLPS) and involves numerous proteins containing intrinsically disordered regions. Whether SOD1, which is also associated with SGs and whose aggregation is associated with Amyotrophic lateral sclerosis (ALS), can independently undergo LLPS, is not known. SOD1 is a metalloenzyme which is stabilized by the metal co-factor - Zn. In this work, we utilize experimental and simulation techniques to highlight the modulation of SOD1 LLPS propensity in a Zn-dependent manner due to underlying conformational transitions between folded and partially disordered states. Our work establishes a link between SOD1 LLPS and aggregation, which is relevant to ALS pathogenesis.