Point mutations in transactive response DNA-binding protein 43 (TDP-43)'s N-terminal domain compromise its stability, dimerization and functions

Abstract

Transactive response DNA-binding protein 43 (TDP-43) performs multiple tasks in mRNA processing, transport and translational regulation, but it also forms aggregates implicated in amyotrophic lateral sclerosis (ALS). TDP-43's N-terminal domain (NTD) is important for these activities and dysfunctions; however, there is an open debate about whether or not it adopts a specifically folded, stable structure. Here, we studied NTD mutations designed to destabilize its structure utilizing NMR and fluorescence spectroscopies, analytical ultracentrifugation, splicing assays and in cell microscopy. The substitutions Val 31>Arg and Thr 32>Arg abolished TDP 43 activity in splicing and aggregation processes and even the rather mild Leu 28>Ala mutation severely destabilized the NTD, drastically reducing TDP-43's in vitro splicing activity and inducing aberrant localization and aggregation in cells. These findings strongly support the idea that a stably folded NTD is essential for correct TDP-43 function. The stably folded NTD also promotes dimerization which is pertinent to the protein's activities and pathological aggregation, and we present an atomic-level structural model for the TDP-43 dimer based on NMR data. Leu 27 is evolutionarily well conserved even though it is exposed in the monomeric NTD. We found here that Leu 27 is buried in the dimer and that the Leu 27>Ala mutation promotes monomerization. In conclusion, our study sheds light on the structural and biological properties of the TDP-43 NTD, indicating that the NTD must be stably folded for TDP-43's physiological functions, and has implications for understanding the mechanisms promoting pathological aggregation of this protein.