Protein folding studies of human superoxide dismutase and ALS associated mutants

Abstract: Proteins are among the most abundant biological macromolecules. The cellular machinery is coupled to exact structural shape and properties of the more than 100 000 different proteins. Still, proteins can sometimes completely change their character and as a result trigger neuro degenerative disease. Exactly what happens is yet poorly understood but misfolding and aggregation leading to toxic gain of function is probable causes, i.e. the protein adopts new noxious properties. In 1993 the protein superoxide dismutase (SOD) was found to be associated with the neuro degenerative disease ALS. Up to date more than 100 mutations in SOD have been associated with ALS. However, the mutations are scattered all over the structure and no common denominator for the disease mechanism has been found.This work has been focused on the molecular mechanism of the toxic gain-of - function of mutant SOD from the perspective of protein folding and structural stability. To facilitate the studies of SOD and its ALS associated mutations, an expression system resulting in increased copper content was developed. Coexpression with the copper chaperone for superoxide dismutase (yCCS) leads to increased expression levels, especially for the destabilised ALS mutants. Through thermodynamic studies, I show that with the exception of the most disruptive mutations the holo protein is only marginally destabilised, whereas all mutations show a pronounced destabilisation on the apo protein. Kinetic studies suggest further that the dimeric apoSOD folds via a three-state process where the dimerisation proceeds via a marginally stable monomer. The apoSOD monomer folds by a two-state process. The disulphide bond is not critical for the folding of the apoSOD monomer although it contributes significantly to its stability. Interestingly, in the absence of metals, reduction of the disulphide bond prevents the formation of the dimer. A mutation can affect the protein stability in various ways: either from destabilisation of the monomer (case 1), weakening of the dimer interface (case 2) or, in the worst case, from a combination of both (case 1+2). Thus, therapeutic strategies to prevent the noxious effects of mutant SOD must include both mechanisms. An important finding in this study is that we can see a correlation between the stability for each mutation and the mean survival time. This could be an opening in the development of therapeutic substances that counteract the defect in SOD upon mutation.