Subcellular membrane-less assemblies are a reinvigorated area study in biology with spirited scientific discussions on the forces between the low-complexity protein domains within these assemblies. To illuminate these forces we determined atomic structures of five segments of protein low-complexity domains associated with membrane-less assemblies. Their common structural feature is the stacking of segments into kinked β-sheets which pair into protofilaments. Unlike steric zippers of amyloid fibrils, the kinked sheets interact weakly through polar atoms and aromatic sidechains. By computationally threading the human proteome on our kinked structures, we identified hundreds of low-complexity segments potentially capable of forming such interactions. These segments are found in proteins as diverse as RNA binders, nuclear pore proteins, and keratins, known to form networks and localize to membrane-less assemblies.

The normally soluble TAR DNA Binding Protein 43 (TDP-43) is found aggregated both in reversible stress granules and irreversible pathogenic amyloid. In TDP-43, the low complexity domain (LCD) is believed to be involved in both types of aggregation. To discover the structural origins of these two modes of β-sheet rich aggregation, we have determined ten structures of segments of the LCD of human TDP-43. Six of these segments form steric zippers characteristic of the spines of pathogenic amyloid fibrils; four others form LARKS, the labile amyloid-like interactions characteristic of protein hydrogels and proteins found in membrane-less organelles, including stress granules. Supporting a hypothetical pathway from reversible to irreversible amyloid aggregation, we found that familial ALS variants of TDP-43 convert LARKS to irreversible aggregates. Our structures suggest how TDP-43 adopts both reversible and irreversible β-sheet aggregates, and the role of mutation in the possible transition of reversible to irreversible pathogenic aggregation.

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