Heteroaggregation of Wild-Type and ALS Mutant SOD1

The presence of wild-type (WT) Cu, Zn superoxide dismutase-1 (SOD1) can increase the toxicity of mutant SOD1 proteins linked to amyotrophic lateral sclerosis (ALS). The mechanism of synergy is unclear but might involve interactions between WT and mutant SOD1 in native or non-native states. One unanswered question is will the diverse rates of mutant SOD1 homofibrillization converge in the presence of WT SOD1? To answer this question, we assessed the coaggregation of mutant and WT SOD1 <i>in vitro</i>, including (i) how WT SOD1 affected the formation rate and stability of mutant fibrils and (ii) the proximity of WT and mutant SOD1 in heterofibrils. For most mutations studied, the presence of WT SOD1 slowed nucleation and propagation of mutant fibrils while increasing fibril thermostability. The D90A SOD1 protein was one exception: WT SOD1 had a nearly negligible effect on its rate of nucleation. The cross-seeding of soluble mutant SOD1 with WT fibrils (and of soluble WT SOD1 with mutant fibrils) suggests that both proteins can occupy the same fibril. Mass spectrometry of heterofibrils treated with an NHS-ester cross-linker (∼8 Å) suggested that WT and E100G mutant SOD1 are colocalized in heterofibrils, possibly stacked in an alternating configuration.