Direct Observation Of Beta-Barrel Intermediates In The Self-Assembly Of Toxic Sod1(28-38) And Absence In Nontoxic Glycine Mutants

Soluble low-molecular-weight oligomers formed during the early stage of amyloid aggregation are considered the major toxic species in amyloidosis. The structure-function relationship between oligomeric assemblies and the cytotoxicity in amyloid diseases are still elusive due to the heterogeneous and transient nature of these aggregation intermediates. To uncover the structural characteristics of toxic oligomeric intermediates, we compared the self-assembly dynamics and structures of SOD1(28-38), a cytotoxic fragment of the superoxide dismutase 1 (SOD1) associated with the amyotrophic lateral sclerosis, with its two nontoxic mutants G33V and G33W using molecular dynamics simulations. Single-point glycine substitutions in SOD1(28-38) have been reported to abolish the amyloid toxicity. Our simulation results showed that the toxic SOD1(28-38) and its nontoxic mutants followed different aggregation pathways featuring distinct aggregation intermediates. Specifically, wild-type SOD1(28-38) initially selfassembled into random-coil-rich oligomers, among which fibrillar aggregates composed of well-defined curved single-layer beta-sheets were nucleated via coil-to-sheet conversions and the formation of beta-barrels as intermediates. In contrast, the nontoxic G33V/G33W mutants readily assembled into small beta-sheet-rich oligomers and then coagulated with each other into cross-beta fibrils formed by twolayer beta-sheets without forming beta-barrels as the intermediates. The direct observation of beta-barrel oligomers during the assembly of toxic SOD1(28-38 )fragments but not the nontoxic glycine-substitution mutants strongly supports beta-barrels as the toxic oligomers in amyloidosis, probably via interactions with the cell membrane and forming amyloid pores. With well-defined structures, the beta-barrel might serve as a novel therapeutic target against amyloid-related diseases.