Next, to examine whether the ATP synthase ␤ subunit RNAi enhances the parkin mutant phenotype, the ATP synthase ␤ subunit was knocked down in park 1 mutant muscles. As shown in Fig. 5F, we observed an increase in ␣/␤-tubulin accumulation. Moreover, the levels of Hsp70/Hsc70 and phosphorylated eIF2␣ in the park 1 mutant were further increased by RNAi knockdown of the ATP synthase ␤ subunit (Fig. 5G). These **data** indicate that down-regulation of ATP synthase activity in the park 1 mutant results in a more severe phenotype compared with that of park 1 single mutants.