Requirement for lysine-19 of the yeast mitochondrial ATPase inhibitor for the stability of the inactivated inhibitor-F1Fo complex at higher pH

The ATPase inhibitor is a regulatory subunit of mitochondrial ATP synthase. In this study, the role of Lys19 of the yeast ATPase inhibitor was examined by site-directed mutagenesis. Two amino acids (Gln and Glu) were substituted for the Lys19. The purified mutant inhibitor (Lys19-->Gln) had similar ATPase inhibitory activity to that of the wild-type inhibitor at pH 6.5, but was less active at pH 7.4. ATP synthesis in mutant mitochondria was normally activated by the addition of ADP and succinate, but the inactivated ATPase complex in the mutant mitochondria was activated more readily than that in control cells by raising pH. These results show that Lys19 of the yeast ATPase inhibitor is not essential for ATPase inhibitory activity, but increases the stability of the inhibitor-F(1)Fo complex at higher pH.