Mapping the critical residues for hVDAC3 activity: electrophysiological study and phenotypic assays in yeast cells of hVDAC3 cysteine mutants

Mutations in superoxide dismutase 1 gene (hSOD1) represent thefirst cause of familial form of Amyotrophic Lateral Sclerosis.Most of ALS-linked SOD1 mutants still show enzymatic activity,suggesting that the SOD1-mediated disease is caused by the gainof some unknown toxic properties, such as impairment of mito-chondrial function. As observed in affected cells, mitochondriaappear enlarged, vacuolated and disorganized [1]; in addition, themost diffused mutants SOD1, G93A and G85R, have been foundassociated with the outer mitochondrial membranes (OMM) [2],suggesting that SOD1 aggregates may be responsible of the mito-chondrial degeneration. As recently demonstrated, this SOD1accumulation on OMM is made possible by the presence of theVoltage-Dependent Anion Channel isoform 1 (VDAC1) [3].VDAC1 is the main pore-forming protein of OMM, directlyinvolved in metabolic cross-talk between mitochondria and cyto-plasm [4]. VDAC1 sequence is highly conserved in eukaryotes:e.g., a high level of similarity was found between VDAC1 andpor1, its homologous in yeast S. cerevisiae. It has been shown thatexpression of corresponding ALS-linked yeast SOD1 mutantG93A, in DSod1 strain, promotes accumulation of this protein onOMM [5], underlining the suitability of yeast to study the distri-bution of mutants SOD1 between cytosol and mitochondria. Tobetter understand the role of VDAC in mitochondrial impairmentSOD1-mediated, the yeast strain lacking of endogenous porin(Dpor1) was used in this work. As reported, Dpor1 is not able togrow in a not fermentable carbon source (glycerol) at 37°C, whileVDAC1 and 2 complement the growth defect. Exploiting this fea-ture, we analyzed the effect of co-presence of human forms ofVDAC and SOD1 proteins on yeast phenotype. Our preliminarydata show that no effect on yeast growth was observed with fer-mentable glucose source; on the contrary, with glycerol, the over-expression of human wild-type SOD1 or misfolded G85R in pres-ence of VDAC1 strongly inhibits yeast growth. Surprisingly, thehuman SOD1 mutants G93A and G37R, are able to restore andameliorate yeast growth. These data highlight how the activity ofSOD1 proteins are linked to the mitochondrial metabolism andare mediated by VDAC. Use of VDAC1 mutants or chimeras asin [6] has allowed us to propose the VDAC regions involved inthe process.