Exploring the Impact of Physiological C-Terminal Truncation on α-Synuclein Conformations to Unveil Mechanisms Regulating Pathological Aggregation.

Emerging evidence suggests that physiological C-terminal truncation of α-synuclein (αS) plays a critical role in regulating liquid-liquid phase separation and promoting amyloid aggregation, processes implicated in neurodegenerative diseases such as Parkinson's disease (PD). However, the molecular mechanisms through which C-terminal truncation influences αS conformation and modulates its aggregation remain poorly understood. In this study, we investigated the impact of C-terminal truncation on αS conformational dynamics by comparing full-length αS_1-140 with truncated αS_1-103 monomers using atomistic discrete molecular dynamics simulations. Our findings revealed that both αS_1-140 and αS_1-103 primarily adopted helical conformations around residues 7-32, while residues 36-95, located in the second half of the N-terminal and NAC domains, predominantly formed a dynamic β-sheet core. The C-terminus of αS_1-140 was largely unstructured and dynamically wrapped around the β-sheet core. While residues 1-95 exhibited similar secondary structure propensities in both αS_1-140 and αS_1-103, the dynamic capping by the C-terminus in αS_1-140 slightly enhanced β-sheet formation around residues 36-95. In contrast, key aggregation-driving regions (residues 2-9, 36-42, 45-57, and 68-78) were dynamically shielded by the C-terminus in αS_1-140, reducing their exposure and potentially preventing interpeptide interactions that drive aggregation. C-terminal truncation, on the other hand, increased the exposed surface area of these aggregation-prone regions, thereby enhancing interpeptide interactions, phase separation, and amyloid aggregation. Overall, our simulations provide valuable insights into the conformational effects of C-terminal truncation on αS and its role in promoting pathological aggregation.