ATP-binding affinity of the ε subunit of thermophilic F(1)-ATPase under label-free conditions

The ε subunits of several bacterial F(1)-ATPases bind ATP. ATP binding to the ε subunit has been shown to be involved in the regulation of F(1)-ATPase from thermophilic Bacillus sp. PS3 (TF(1)). We previously reported that the dissociation constant for ATP of wild-type ε subunit of TF(1) at 25 °C is 4.3 μM by measuring changes in the fluorescence of the dye attached to the ε subunit (Kato, S. et al. (2007) J. Biol. Chem.282, 37618). However, we have recently noticed that this varies with the dye used. In this report, to determine the affinity for ATP under label-free conditions, we have measured the competitive displacement of 2′(3′)-O-N′-methylaniloyl-aminoadenosine-5′-triphosphate (Mant-ATP), a fluorescent analog of ATP, by ATP. The dissociation constant for ATP of wild-type ε subunit of TF(1) at 25 °C was determined to be 0.29 μM, which is one order of magnitude higher affinity than previously reported values.