Transient structural distortion of metal-free Cu/Zn superoxide dismutase triggers aberrant oligomerization

doi:10.1073/pnas.0907387106

Transient structural distortion of metal-free Cu/Zn superoxide dismutase triggers aberrant oligomerization

^aCenter for Molecular Protein Science, Biophysical Chemistry, Lund University, SE-22100 Lund, Sweden;
^bStructural Biology and NMR Laboratory, Department of Biology, University of Copenhagen, DK-2200 Copenhagen, Denmark; and
^cDepartment of Biochemistry and Biophysics, The Arrhenius Laboratories for Natural Sciences, Stockholm University, SE-10691 Stockholm, Sweden

↵¹Present address: Department of Cellular and Molecular Pharmacology, University of California, 1700 4th Street, San Francisco, CA 94158.
↵²Present address: Department of Molecular Structural Biology, University of Göttingen, Justus-von-Liebig-Weg 11, 37077 Göttingen, Germany.

Edited by Alan Fersht, University of Cambridge, Cambridge, UK and approved August 31, 2009 (received for review July 2, 2009)

Abstract

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease linked to the misfolding of Cu/Zn superoxide dismutase (SOD1). ALS-related defects in SOD1 result in a gain of toxic function that coincides with aberrant oligomerization. The structural events triggering oligomerization have remained enigmatic, however, as is the case in other protein-misfolding diseases. Here, we target the critical conformational change that defines the earliest step toward aggregation. Using nuclear spin relaxation dispersion experiments, we identified a short-lived (0.4 ms) and weakly populated (0.7%) conformation of metal-depleted SOD1 that triggers aberrant oligomerization. This excited state emanates from the folded ground state and is suppressed by metal binding, but is present in both the disulfide-oxidized and disulfide-reduced forms of the protein. Our results pinpoint a perturbed region of the excited-state structure that forms intermolecular contacts in the earliest nonnative dimer/oligomer. The conformational transition that triggers oligomerization is a common feature of WT SOD1 and ALS-associated mutants that have widely different physicochemical properties. But compared with WT SOD1, the mutants have enhanced structural distortions in their excited states, and in some cases slightly higher excited-state populations and lower kinetic barriers, implying increased susceptibility to oligomerization. Our results provide a unified picture that highlights both (i) a common denominator among different SOD1 variants that may explain why diverse mutations cause the same disease, and (ii) a structural basis that may aid in understanding how different mutations affect disease propensity and progression.

Footnotes

³To whom correspondence should be addressed. E-mail: mikael.akke{at}bpc.lu.se
Author contributions: K.T., M.H.S., M.O., and M.A. designed research; K.T., M.H.S., E.S., L.C.C., and G.S. performed research; K.T. and M.O. contributed new reagents/analytic tools; K.T., M.H.S., E.S., G.S., and M.A. analyzed data; and K.T., M.H.S., and M.A. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: Backbone chemical shift assignments for WT apo-SOD1 and mutants A4V, G85R, and D90A have been deposited at the BioMagResBank, www.bmrb.wisc.edu (accession nos. 15711, 15712, 15713, and 15714).
This article contains supporting information online at www.pnas.org/cgi/content/full/0907387106/DCSupplemental.