The alpha 3 beta 3 complex, the catalytic core of F1-ATPase.

The alpha 3 beta 3 complex was reconstituted from alpha and beta subunits of the thermophilic bacterium PS3 F1-ATPase (TF1) and then isolated. It is less stable at high and low temperatures than TF1, and the complex dissociates into subunits during native polyacrylamide gel electrophoresis. The alpha 3 beta 3 complex has about 20% of the ATPase activity of TF1. Its enzymic properties are similar to those of the native TF1, exhibiting similar cooperative kinetics as a function of ATP concentration, similar substrate specificity for nucleotide triphosphates, and the presence of two peaks in its temperature-activity profile. Differing from TF1, the ATPase activity of the alpha 3 beta 3 complex is insensitive to N3- inhibition, its divalent cation specificity is less stringent, and its optimum pH shifts to the alkaline side. The addition of the gamma subunit to the alpha 3 beta 3 complex leads to the formation of the alpha 3 beta 3 gamma complex, indicating that the alpha 3 beta 3 complex is an intermediate in the process of assembly of the holoenzyme from each subunit. These results definitely show that the essential structure for eliciting the ATPase activity of F1-ATPase is trimeric alpha beta pairs and that the kinetic cooperativity of the F1-ATPase is an inherent property of this trimeric structure but is not due to the presence of single-copy subunits. In this sense, the alpha 3 beta 3 complex is the catalytic core of F1-ATPase.