The isolated E; subunit of F-1-ATPase from thermophilic Bacillus PS3 (TF1) binds ATP [Y. Kato-Yamada, M. Yoshida, J. Biol. Chem. 278 (2003) 36013]. The obvious question is whether the ATP binding concern with the regulation of ATP synthase activity or not. If so, the F subunit even in the ATP synthase complex should have the ability to bind ATP. To check if the ATP binding to the F subunit within the ATP synthase complex may occur, the gamma epsilon sub-complex of TF1 was prepared and ATP binding was examined. The results clearly showed that the gamma epsilon sub-complex can bind ATP. (c) 2006 Elsevier Inc. All rights reserved.