Investigating Curvature Sensing by the Nt17 Domain of Huntingtin Protein

Nt17, the N-terminal domain of the huntingtin protein (htt), has garnered significant attention for its role in htt's membrane binding and aggregation processes. Previous studies have identified a nuclear export sequence within the Nt17 domain and demonstrated its localization at various cellular organelles. Recent evidence suggests that, like other amphipathic helices, Nt17 can sense and preferentially bind to curved membranes. Gaining deeper insight into this behavior is essential to fully understand the function of this domain. In this study, we combine coarse-grained molecular dynamics simulations with circular dichroism (CD) spectroscopy to investigate the mechanism behind Nt17's curvature sensing. We generated a unique hemispherical-planar membrane model, where 36% of the upper leaflet surface is curved, allowing us to evaluate Nt17's binding preferences. Our findings show that Nt17 exhibits a strong preference for curved regions, with approximately 78 ± 7% of peptides binding to these areas. This interaction is primarily mediated by the terminal Phe residues, indicating that Nt17's curvature sensing is driven by its ability to detect lipid packing defects. Furthermore, Nt17 not only senses these defects but also amplifies them by coalescing into smaller pockets. Mutating the Phe residues to methionine, a smaller hydrophobic residue, significantly reduces Nt17's curvature sensitivity, resulting in equal binding to both curved and planar regions. CD spectroscopy corroborates these results, showing that Nt17 binds more strongly to highly curved small unilamellar vesicles (SUVs) compared to larger, less curved large unilamellar vesicles (LUVs).