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Molecular Biology of the Cell (1)
Caplow, Michael (2)
Fee, Lanette (2)
Dokholyan, Nikolay V. (1)
Guidotti, Guido (1)
Proctor, Elizabeth A. (1)
Redler, Rachel L. (1)
Wilcox, Kyle C. (1)
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Glutathionylation at Cys-111 Induces Dissociation of Wild Type and FALS Mutant SOD1 Dimers
Redler, Rachel L.
Wilcox, Kyle C.
Proctor, Elizabeth A.
Dokholyan, Nikolay V.
Mutation of the ubiquitous cytosolic enzyme Cu/Zn superoxide dismutase (SOD1) is hypothesized to cause familial amyotrophic lateral sclerosis (FALS) through structural destabilization leading to misfolding and aggregation. Considering the late onset of symptoms as well as the phenotypic variability among patients with identical SOD1 mutations, it is clear that nongenetic factor(s) impact ALS etiology and disease progression. Here we examine the effect of Cys-111 glutathionylation, a physiologically prevalent post-translational oxidative modification, on the stabilities of wild type SOD1 and two phenotypically diverse FALS mutants, A4V and I112T. Glutathionylation results in profound destabilization of SOD1WT dimers, increasing the equilibrium dissociation constant Kd to ~10−20 μM, comparable to that of the aggressive A4V mutant. SOD1A4V is further destabilized by glutathionylation, experiencing an ~30-fold increase in Kd. Dissociation kinetics of glutathionylated SOD1WT and SOD1A4V are unchanged, as measured by surface plasmon resonance, indicating that glutathionylation destabilizes these variants by decreasing association rate. In contrast, SOD1I112T has a modestly increased dissociation rate but no change in Kd when glutathionylated. Using computational structural modeling, we show that the distinct effects of glutathionylation on different SOD1 variants correspond to changes in composition of the dimer interface. Our experimental and computational results show that Cys-111 glutathionylation induces structural rearrangements that modulate stability of both wild type and FALS mutant SOD1. The distinct sensitivities of SOD1 variants to glutathionylation, a modification that acts in part as a coping mechanism for oxidative stress, suggest a novel mode by which redox regulation and aggregation propensity interact in ALS.
Dissociation of the Tubulin Dimer Is Extremely Slow, Thermodynamically Very Unfavorable, and Reversible in the Absence of an Energy Source
Molecular Biology of the Cell
The finding that exchange of tubulin subunits between tubulin dimers (α-β + α′β′ ↔ α′β + αβ′) does not occur in the absence of protein cofactors and GTP hydrolysis conflicts with the assumption that pure tubulin dimer and monomer are in rapid equilibrium. This assumption underlies the many physical chemical measurements of the Kd for dimer dissociation. To resolve this discrepancy we used surface plasmon resonance to determine the rate constant for dimer dissociation. The half-time for dissociation was ∼9.6 h with tubulin-GTP, 2.4 h with tubulin-GDP, and 1.3 h in the absence of nucleotide. A Kd equal to 10−11 M was calculated from the measured rate for dissociation and an estimated rate for association. Dimer dissociation was found to be reversible, and dimer formation does not require GTP hydrolysis or folding information from protein cofactors, because 0.2 μM tubulin-GDP incubated for 20 h was eluted as dimer when analyzed by size exclusion chromatography. Because 20 h corresponds to eight half-times for dissociation, only monomer would be present if dissociation were an irreversible reaction and if dimer formation required GTP or protein cofactors. Additional evidence for a 10−11 M Kd was obtained from gel exclusion chromatography studies of 0.02–2 nM tubulin-GDP. The slow dissociation of the tubulin dimer suggests that protein tubulin cofactors function to catalyze dimer dissociation, rather than dimer assembly. Assuming N-site-GTP dissociation is from monomer, our results agree with the 16-h half-time for N-site GTP in vitro and 33 h half-life for tubulin N-site-GTP in CHO cells.
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