Conversion of the Native N-Terminal Domain of TDP-43 into a Monomeric Alternative Fold with Lower Aggregation Propensity

TAR DNA-binding protein 43 (TDP-43) forms intraneuronal cytoplasmic inclusions associated with amyotrophic lateral sclerosis and ubiquitin-positive frontotemporal lobar degeneration. Its N-terminal domain (NTD) can dimerise/oligomerise with the head-to-tail arrangement, which is essential for function but also favours liquid-liquid phase separation and inclusion formation of full-length TDP-43. Using various biophysical approaches, we identified an alternative conformational state of NTD in the presence of Sulfobetaine 3-10 (SB3-10), with higher content of alpha-helical structure and tryptophan solvent exposure. NMR shows a highly mobile structure, with partially folded regions and beta-sheet content decrease, with a concomitant increase of alpha-helical structure. It is monomeric and reverts to native oligomeric NTD upon SB3-10 dilution. The equilibrium GdnHCl-induced denaturation shows a cooperative folding and a somewhat lower conformational stability. When the aggregation processes were compared with and without pre-incubation with SB3-10, but at the identical final SB3-10 concentration, a slower aggregation was found in the former case, despite the reversible attainment of the native conformation in both cases. This was attributed to protein monomerization and oligomeric seeds disruption by the conditions promoting the alternative conformation. Overall, the results show a high plasticity of TDP-43 NTD and identify strategies to monomerise TDP-43 NTD for methodological and biomedical applications.

Automated summary

TAR DNA-binding protein 43 (TDP-43) has been associated with amyotrophic lateral sclerosis and frontotemporal lobar degeneration. The N-terminal domain (NTD) of TDP-43 plays a crucial role in its function and the formation of intraneuronal inclusions. The study identified an alternative conformation of NTD in the presence of SB3-10, which led to a more α-helical structure. The results also showed that the aggregation of TDP-43 was slower when pre-incubated with SB3-10, indicating the disruption of oligomeric seeds by the alternative conformation conditions. These findings provide insights into the plasticity of TDP-43 NTD and suggest strategies for monomerising TDP-43 NTD for various applications.