ALS Mutations Disrupt Phase Separation Mediated by a Helical Structure in the TDP 43 Low Complexity C Terminal Domain.
RNA binding protein TDP 43 mediates essential RNA processing but forms cytoplasmic neuronal inclusions via its C terminal domain (CTD) in amyotrophic lateral sclerosis (ALS) It remains unclear if aggregated TDP 43 is neurotoxic and if ~50 ALS associated missense mutations in TDP 43 CTD promote aggregation, or if loss of normal function plays a role in disease. Recent work points to the ability of related proteins to assemble into functional phase separated ribonucleoprotein granules via their structurally disordered prion like domains. Here, we provide atomic details on the structure and assembly of the low complexity CTD of TDP 43 into liquid liquid phase separated in vitro granules and demonstrate that ALS associated variants disrupt interactions within granules. Using nuclear magnetic resonance spectroscopy, simulation, and microscopy, we find that a subregion cooperatively but transiently folds into a helix that mediates TDP 43 phase separation. ALS associated mutations disrupt phase separation by inhibiting interaction and helical stabilization. Therefore, ALS associated mutations can disrupt TDP 43 interactions, affecting function beyond encouraging aggregation.
Structure