Detection of an Intermediate in the Unfolding Process of the N-Terminal Domain of TDP-43

TAR DNA-binding protein 43 (TDP-43) is a nuclear protein accumulating in intraneuronal cytoplasmic inclusions associated with amyotrophic lateral sclerosis, frontotemporal lobar degeneration with tau-negative/ubiquitin-positive inclusions, and limbic-predominant age-related TDP-43 encephalopathy. Oligomerization of full-length TDP-43, driven by its N-terminal domain (NTD), is essential for its function, but aberrant self-assembly also promotes liquid-liquid phase separation and formation of solid inclusions. Building on recent all-atom molecular dynamics simulations and using various biophysical approaches, we identified a partially unfolded state accumulating during unfolding of TDP-43 NTD, before the major energy barrier of unfolding is crossed. Intrinsic fluorescence spectroscopy coupled to a stopped-flow device at high urea concentration reveals that the intermediate state has a fluorescence emission distinct from those of the native and unfolded states and forms within the 14 ms dead time. Conventional fluorescence spectroscopy shows it still accumulates at moderate urea concentration. Circular dichroism and H/D exchange results show a species with an intermediate content of secondary structure and a distorted β-sheet, whereas SYPRO orange fluorescence indicates an open conformation with more exposed hydrophobic regions compared to the native state. Importantly, this intermediate is observed even at low protein concentration, when TDP-43 NTD is largely monomeric, indicating that its formation is independent of the initial TDP-43 NTD oligomeric state. Dynamic light scattering at high protein concentration shows that the intermediate is a partially folded dimer. The intermediate forms upon chemical denaturation and does not occur under thermal unfolding. Overall, the findings highlight the presence of one more partially folded state for TDP-43 NTD, underlining its high structural plasticity and suggesting that its distinct unfolding pathway may play a critical role in both its functional and pathological behaviors.