Inhibition of ATP hydrolysis by thermoalkaliphilic F1Fo-ATP synthase is controlled by the C terminus of the epsilon subunit

The F1Fo-ATP synthases of alkaliphillic bacteria exhibit latent ATPase activity, and for the thermoalkaliphile Bacillus sp. strain TA2.A1, this activity is intrinsic to the F, moiety. To study the mechanism of ATPase inhibition, we developed a heterologous expression system in Escherichia coli to produce TA2F(1), complexes from this thermoalkaliphile. Like the native F1Fo-ATP synthase, the recombinant TA2F, was blocked in ATP hydrolysis activity, and this activity was stimulated by the detergent lauryldimethylamine oxide. To determine if the C-terminal domain of the e subunit acts as an inhibitor of ATPase activity and if an electrostatic interaction plays a role, a TA2F, mutant with either a truncated E subunit [i.e., TA2F(1)(epsilon(Delta C))] or substitution of basic residues in the second et-helix of e with nonpolar alanines [i.e., TA2F,(epsilon(6A))] was constructed. Both mutants showed ATP hydrolysis activity at low and high concentrations of ATE Treatment of the purified F1Fo-ATP synthase and TA2F(1)(epsilon(WT)) complex with proteases revealed that the E subunit was resistant to, was completely degraded by trypsin, indicating proteolytic digestion. In contrast, the E subunit of TA2F(1)(epsilon(6A)) that the C-terminal arm was in a conformation where it was no longer protected from proteollytic digestion. In addition, ATPase activity was not further activated by protease treatment when compared to the untreated control, supporting the observation that E was responsible for inhibition of ATPase activity. To study the effect of the alanine substitutions in the E subunit in the entire holoenzyme, we reconstituted recombinant TA2F(1) complexes with F-1-stripped native membranes of strain TA2.A1. The reconstituted TA2F(o)F(1)(epsilon(WT)) was blocked in ATP hydrolysis and exhibited low levels of ATP-driven proton pumping consistent with the F1Fo-ATP synthase in native membranes. Reconstituted TA2F(o)F(1) (epsilon(6A)) exhibited ATPase activity that correlated with increased ATP-driven proton pumping, confirming that the epsilon subunit also inhibits ATPase activity of TA2F(o)F(1).