Role of the epsilon Subunit of Thermophilic F sub(1)-ATPase as a Sensor for ATP

The epsilon subunit of F sub(1)-ATPase from the thermophilic Bacillus PS3 (TF sub(1)) has been shown to bind ATP. The precise nature of the regulatory role of ATP binding to the epsilon subunit remains to be determined. To address this question, 11 mutants of the epsilon subunit were prepared, in which one of the basic or acidic residues was substituted with alanine. ATP binding to these mutants was tested by gel-filtration chromatography. Among them, four mutants that showed no ATP binding were selected and reconstituted with the alpha sub(3) beta sub(3) gamma complex of TF sub(1). The ATPase activity of the resulting alpha sub(3) beta sub(3) gamma epsilon complexes was measured, and the extent of inhibition by the mutant epsilon subunits was compared in each case. With one exception, weaker binding of ATP correlated with greater inhibition of ATPase activity. These results clearly indicate that ATP binding to the epsilon subunit plays a regulatory role and that ATP binding may stabilize the ATPase-active form of TF sub(1) by fixing the epsilon subunit into the folded conformation.