The inhibitor protein IF1 from mammalian mitochondria inhibits ATP hydrolysis but not ATP synthesis by the ATP synthase complex
Joe Carroll, Ian N. Watt, Charlotte J Wright, Shujing Ding, Ian M. Fearnley, John E. Walker
Abstract
The hydrolytic activity of the ATP synthase in bovine mitochondria is inhibited by a protein called IF 1 , but bovine IF 1 has no effect on the synthetic activity of the bovine enzyme in mitochondrial vesicles in the presence of a proton motive force. In contrast, it has been suggested based on indirect observations that human IF I inhibits both the hydrolytic and synthetic activities of the human ATP synthase, and that the activity of human IF 1 is regulated by the phosphorylation of serine-14 of mature IF 1 . Here, we have made both human and bovine IF 1 which are 81 and 84 amino acids long, respectively, and identical in 71.4% of their amino acids, and have investigated their inhibitory effects on the hydrolytic and synthetic activities of ATP synthase in bovine sub-mitochondrial particles. Over a wide range of conditions, including physiological conditions, both human and bovine IF 1 are potent inhibitors of ATP hydrolysis, with no effect on ATP synthesis. Also, substitution of serine-14 with phosphomimetic aspartic and glutamic acids had no effect on inhibitory properties, and serine-14 is not conserved throughout mammals. Therefore, it is unlikely that the inhibitory activity of mammalian IF 1 is regulated by phosphorylation of this residue.