Structural Comparison of the Human G93A Mutant SOD1 to the Wild-type SOD1 Filaments

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal neurodegenerative disease characterized by Cu, Zn-superoxide dismutase (SOD1) misfolding and aggregation, particularly in familial cases. The G93A mutation in SOD1, strongly associated with familial ALS, is widely studied in transgenic mouse models of the disease. In this study, we investigated the filament structure of the G93A mutant SOD1 using cryo-electron microscopy. The resulting fibrils consisted of a single protofilament with a left-handed helical twist, closely resembling those formed by wild-type (WT) SOD1 under identical conditions. Self- and cross-seeding experiments promoted filament formation in both WT and G93A mutant SOD1, compared to the no-seed condition. Notably, the G93A mutant exhibited significantly higher susceptibility to proteolysis in its native state than WT SOD1. Mass spectrometry analysis suggested that the structurally disordered electrostatic loop acts as a key common intermediate structure in filament formation for both WT and G93A mutant SOD1. These findings suggest that shared filament formation pathways underlie the aggregation of both WT and G93A mutant SOD1, providing new insights into the molecular mechanisms contributing to ALS pathogenesis.