DmGSTO1 Restores Mitochondrial ATP Synthase Activity in park 1 Mutants-We next examined whether the catalytic activity of DmGSTO1A was critical for the rescue the park 1 mutant phenotypes. GSTOs have a unique cysteine at their active site that binds to GSH (27). We constructed a catalytically inactive form of DmGSTO1A, DmGSTO1A C31A , in which cysteine 31 was mutated to alanine. In contrast to wild type DmGSTO1A, expression of DmGSTO1A C31A in muscle did not rescue the collapsed thorax and downturned wing phenotypes in park 1 mutants (Fig. 4A). Moreover, tubulin accumulation was not suppressed by DmGSTO1A C31A expression in park 1 mutants (Fig. 4B). Our **data** demonstrate that DmGSTO1 catalytic activity is required to rescue the defective phenotypes of park 1 mutants.