Identification of a Misfolded Region in Superoxide Dismutase 1 that is Exposed in Amyotrophic Lateral Sclerosis [Protein Structure and Folding]
August 27th, 2014 by Rotunno, M. S., Auclair, J. R., Maniatis, S., Shaffer, S. A., Agar, J., Bosco, D. A.
Mutations and aberrant post-translational modifications within Cu,Zn-superoxide dismutase (SOD1) cause this otherwise protective enzyme to misfold, leading to amyotrophic lateral sclerosis (ALS). The C4F6 antibody selectively binds misfolded SOD1 in spinal cord tissues from post-mortem human ALS cases as well as from an ALS-SOD1 mouse model, suggesting the C4F6 epitope reports on a pathogenic conformation that is common to misfolded SOD1 variants. To date, the residues and structural elements that comprise this epitope have not been elucidated. Using a chemical cross-linking and mass spectrometry approach, we identified the C4F6 epitope within several ALS-linked SOD1 variants as well as an oxidized form of WT SOD1, supporting the notion that a similar misfolded conformation is shared amongst pathological SOD1 proteins. Exposure of the C4F6 epitope was modulated by the SOD1 electrostatic (loop VII) and zinc binding (loop IV) loops, and correlated with SOD1-induced toxicity in a primary microglia activation assay. Site directed mutagenesis revealed Asp92 and Aps96 as key residues within the C4F6 epitope required for the SOD1/C4F6 binding interaction. We propose that stabilizing the functional loops within SOD1 and/or obscuring the C4F6 epitope are viable therapeutic strategies for treating SOD1-mediated ALS.