Cryo-EM structure of an amyloid fibril formed by full-length human SOD1 reveals its pathological conformational conversion

Abstract

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal neurodegenerative disease characterized by the selective death of motor neurons. Misfolded Cu, Zn-superoxide dismutase (SOD1) has been linked to both familial ALS and sporadic ALS. SOD1 fibrils formed in vitro are able to incorporate into cells, transmit intercellularly, and share toxic properties with ALS inclusions. Here we produced amyloid fibrils in vitro from recombinant, full-length apo human SOD1 under semi-reducing conditions and determined the atomic structure using cryo-EM. The SOD1 fibril consists of a single protofibril with a left-handed helix. The fibril core exhibits a serpentine fold comprising N-terminal segment (residues 3 to 55) and C-terminal segment (residues 86 to 153) with a structural break. The two segments are zipped up by three salt bridge pairs. By comparison with the structure of apo SOD1 dimer, we propose that eight β-strands (to form a β-barrel) and one α-helix in the subunit of apo SOD1 convert into thirteen β-strands stabilized by five hydrophobic cavities in the SOD1 fibril. Our data provide insights into how SOD1 converts between structurally and functionally distinct states.