Homogeneous ε{lunate} bound tightly to the purified Escherichia coli ATPase (ECF1 from which ε{lunate} had been removed and strongly inhibited its ATPase activity. ECF1 containing ε{lunate} had a lower specific activity than ECF1 missing ε{lunate}, provided that the ATPase assay was carried out at relatively high concentrations of enzyme. Antiserum specific for the ε{lunate} subunit stimulated the ATPase, as did diluting the enzyme, apparently by dissociating ε{lunate}. When the ATPase reaction was started by the addition of enzyme, the rate of ATP hydrolysis increased progressively during the first 3 min until a linear steady-state rate was reached. A prior incubation with ATP abolished the lag period and ADP prevented the ATP effect. ECF1 missing ε{lunate} gave a linear rate of ATP hydrolysis without a lag, unless ε{lunate} was rebound to it before the assay. These results suggest that ECF1 as purified is in an inhibited state due to the presence of the ε{lunate} subunit, whose interaction with ECF1 is governed by an equilibrium binding. ATP appears to convert ECF1 to a form which more readily binds and releases ε{lunate}. © 1979.
