Over 100 point mutations in the Cu/Zn superoxide dismutase (SOD) gene have been linked to the neurodegenerative disease amyotrophic lateral sclerosis (ALS). Exactly why these mutations cause ALS is unclear, but abounding explanations fall into two main camps-those that hinge on mutants being catalytically active, and those that do not. A report in today's PNAS from researchers at Umea University, Sweden, strongly supports the latter view.
Senior author Mikael Oliveberg and colleagues characterized ALS-associated SOD mutants containing single amino-acid substitutions in various parts of the protein. The authors found that one thing all mutants had in common is that their apo protein, i.e., the protein alone without its active-site metal ions, is less stable than wild-type apo protein. First author Mikael Lindberg et al. determined this by denaturing the proteins with either guanidium chloride or urea.
The authors found that substituting alanine for glycine at position 93, for example, resulted in complete denaturation of the polypeptide at a concentration of two molar urea, whereas four molar is needed to denature wild-type apo protein. Mutating N-terminal amino acids (valine for alanine at position 4, and phenylalanine for cysteine at postion 6), which are buried in the hydrophobic core of the protein, had even more drastic effects. Interestingly, substituting alanine for cysteine at position 6, a mutation that is not associated with ALS but is found in other, presumably healthy organisms, has no effect on the stability of the apo protein.
The results suggest that destabilization of the immature protein may be a contributing factor to ALS progression. This supports recent data indicating that copper loading of SOD is not required for manifestation of the disease (see related new item). It is also noteworthy that the least stable mutants (i.e., A4V, and C6F) are also associated with faster progression of ALS.—Tom Fagan
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- Lindberg MJ, Tibell L, Oliveberg M. Common denominator of Cu/Zn superoxide dismutase mutants associated with amyotrophic lateral sclerosis: decreased stability of the apo state. Proc Natl Acad Sci U S A. 2002 Dec 24;99(26):16607-12. PubMed.