Mechanism for transforming cytosolic SOD1 into integral membrane proteins of organelles by ALS-causing mutations

• Conformations of membrane-associated wild type and ALS mutant SOD1 determined by NMR.
• An ALS-causing SOD1 mutant inserts into membranes by forming a helical conformation.
• Zinc-depletion converts wild type SOD1 into a membrane-bound helical conformation.
• β-Strands of wild type or mutant SOD1 convert to mostly helices when membrane bound.
• SOD1 association with membranes is mediated by interfacial amphiphilic helices.

Mutations in superoxide dismutase 1 (SOD1) cause familial amyotrophic lateral sclerosis (FALS), while wild-type SOD1 has been implicated in sporadic ALS (SALS). SOD1 mutants are now recognized to acquire one or more toxicities that include their association with mitochondrial and endoplasmic reticulum membranes but the underlying structural mechanism remains unknown. Here we determine NMR conformations of both wild-type and a truncation mutant (L126Z) of SOD1 in aqueous solution and a membrane environment. The truncation mutant (which causes FALS at very low levels, indicating its elevated toxicity) is highly unstructured in solution, failing to adopt the β-barrel SOD1 native structure. Wild-type SOD1 is also highly unstructured upon reduction of disulfides and depletion of zinc. Most remarkably, both mutant and wild type adopt similar, highly-helical conformations in a membrane environment. Thus, either truncation or depletion of zinc is sufficient to eliminate the native β-barrel structure, and transform cytosolic SOD1 into membrane proteins energetically driven by forming amphiphilic helices in membranes. That zinc-deficiency is sufficient to produce a similar transformation in wild-type SOD1 implies that the wild-type and FALS-linked SOD1 mutants may trigger ALS by a common mechanism.

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