Metal migration and subunit swapping in ALS-linked SOD1 Zn2+ transfer between mutant and wild-type occurs faster than the rate of heterodimerization

The heterodimerization of WT Cu, Zn superoxide dismutase-1 (SOD1), and mutant SOD1 might be a critical step in the pathogenesis of SOD1-linked amyotrophic lateral scle-rosis (ALS). Rates and free energies of heterodimerization (Delta GHet) between WT and ALS-mutant SOD1 in mismatched metalation states-where one subunit is metalated and the other is not-have been difficult to obtain. Consequently, the hypothesis that under-metalated SOD1 might trigger misfold-ing of metalated SOD1 by stealing metal ions remains untested. This study used capillary zone electrophoresis and mass spectrometry to track heterodimerization and metal transfer between WT SOD1, ALS-variant SOD1 (E100K, E100G, D90A), and triply deamidated SOD1 (modeled with N26D/N131D/N139D substitutions). We determined that rates of subunit exchange between apo dimers and metalated dimers-expressed as time to reach 30% heterodimer-ranged from t30% = 67.75 +/- 9.08 to 338.53 +/- 26.95 min; free energies of heterodimerization ranged from Delta G(Het) =-1.21 +/- 0.31 to-3.06 +/- 0.12 kJ/mol. Rates and Delta G(Het) values of partially metalated heterodimers were more similar to those of fully metalated heterodimers than apo heterodimers, and largely independent of which subunit (mutant or WT) was metal-replete or metal -free. Mass spectrometry and capillary electrophoresis demon-strated that mutant or WT 4Zn-SOD1 could transfer up to two equivalents of Zn2+ to mutant or WT apo-SOD1 (at rates faster than the rate of heterodimerization). This result suggests that zinc-replete SOD1 can function as a chaperone to deliver Zn2+ to apo-SOD1, and that WT apo-SOD1 might increase the toxicity of mutant SOD1 by stealing its Zn2+.

Automated summary

This study investigated heterodimerization and metal transfer between WT SOD1, ALS-variant SOD1, and deamidated SOD1. The rates and free energies of heterodimerization were determined, and it was found that partially metalated heterodimers have similar properties to fully metalated heterodimers. The study also demonstrated that zinc-replete SOD1 can transfer Zn2+ to apo-SOD1, and suggested that WT apo-SOD1 may increase the toxicity of mutant SOD1 by stealing its zinc.