Structural dissection of amyloid aggregates of TDP‐43 and its C‐terminal fragments TDP‐35 and TDP‐16

The TAR DNA‐binding protein (TDP‐43) self‐assembles into prion‐like aggregates considered to be the structural hallmark of amyotrophic lateral sclerosis and frontotemporal dementia. Here, we use a combination of electron microscopy, X‐ray fiber diffraction, Fourier‐transform infrared spectroscopy analysis, and solid‐state NMR spectroscopy to investigate the molecular organization of different TDP constructs, namely the full‐length TDP‐43 (1–414), two C‐terminal fragments [TDP‐35 (90–414) and TDP‐16 (267–414)], and a C‐terminal truncated fragment (TDP‐43 ∆GaroS2), in their fibrillar state. Although the different protein constructs exhibit similar fibril morphology and a typical cross‐β signature by X‐ray diffraction, solid‐state NMR indicates that TDP‐43 and TDP‐35 share the same polymorphic molecular structure, while TDP‐16 encompasses a well‐ordered amyloid core. We identified several residues in the so‐called C‐terminal GaroS2 (368–414) domain that participates in the rigid core of TDP‐16 fibrils, underlining its importance during the aggregation process. Our findings demonstrate that C‐terminal fragments can adopt a different molecular conformation in isolation or in the context of the full‐length assembly, suggesting that the N‐terminal domain and RRM domains play an important role in the TDP‐43 amyloid transition.