F1-ATPase is an ATP-driven motor in which γε rotates in the α3β3-cylinder. It is attenuated by MgADP inhibition and by the ε subunit in an inhibitory form. The non-inhibitory form of ε subunit of thermophilic Bacillus PS3 F1-ATPase is stabilized by ATP-binding with micromolar Kd at 25 °C. Here, we show that at [ATP] > 2 μM, ε does not affect rotation of PS3 F1-ATPase but, at 200 nM ATP, ε prolongs the pause of rotation caused by MgADP inhibition while the frequency of the pause is unchanged. It appears that ε undergoes reversible transition to the inhibitory form at [ATP] below Kd.
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