Abstract
Superoxide dismutase 1 (SOD1) is an important metalloprotein for cellular oxidative stress defence, that is mutated in familiar variants of Amyotrophic Lateral Sclerosis (fALS). Some mutations destabilize the apo protein, leading to the formation of misfolded, toxic species. The Copper Chaperone for SOD1 (CCS) transiently interacts with SOD1 and promotes its correct maturation by transferring copper and catalyzing disulfide bond formation. By in vitro and in-cell NMR, we investigated the role of the SOD-like domain of CCS (CCS-D2). We showed that CCS-D2 forms a stable complex with zinc-bound SOD1 in human cells, that has a twofold stabilizing effect: it both prevents the accumulation of unstructured mutant SOD1 and promotes zinc binding. We further showed that CCS-D2 interacts with apo-SOD1 in vitro, suggesting that in cells CCS stabilizes mutant apo-SOD1 prior to zinc binding. Such molecular chaperone function of CCS-D2 is novel and its implications in SOD-linked fALS deserve further investigation.
Highlights
Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease characterized by the death of motor neurons in the brain and spinal cord
WT Superoxide dismutase 1 (SOD1) overexpressed in the human cell cytoplasm in the presence of excess zinc is found in a stable intermediate maturation state, i.e. a homodimer with one zinc ion bound to each monomer, and all cysteines reduced ( E,Zn-SOD1SH)[25]
The interaction between SOD1 and Copper Chaperone for SOD1 (CCS) – mediated by D2 – has been extensively investigated before[15,16,17,18,19,20,21,22,23,24], and CCS is known to have a stabilizing effect on the maturation pathway of SOD1: overexpression of CCS in cell cultures and transgenic mice has previously revealed a protective function against the aggregation of mutant SOD133–35
Summary
Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease characterized by the death of motor neurons in the brain and spinal cord. It has been shown that some WT-L fALS SOD1 mutants fail to bind zinc when they are overexpressed in the human cytoplasm, and accumulate as unstructured species that may act as precursors in the pathogenic aggregation pathway[28] This mechanism can be reverted by co-expressing CCS, indicating that the copper chaperone is able to pull the nascent SOD1 molecules towards the correct folding and maturation pathway. In vitro analysis revealed that, in the heterodimer observed in cells, SOD1 is in the zinc-bound form These data suggest that D2 of CCS favours zinc binding to mutant SOD1 by interacting either with the zinc-bound protein and preventing zinc dissociation, or with the apo protein stabilizing it and allowing zinc binding. In favour of the latter hypothesis, we further showed that D2 of CCS is able to interact with apo-SOD1 in vitro, and that CCS acts as a molecular chaperone towards the most immature state of SOD1
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