The Structural Basis for Unidirectional Rotation of Thermoalkaliphilic F1-ATPase

SummaryThe ATP synthase of the thermoalkaliphilic Bacillus sp. TA2.A1 operates exclusively in ATP synthesis direction. In the crystal structure of the nucleotide-free α3β3γɛ subcomplex (TA2F1) at 3.1 Å resolution, all three β subunits adopt the open βE conformation. The structure shows salt bridges between the helix-turn-helix motif of the C-terminal domain of the βE subunit (residues Asp372 and Asp375) and the N-terminal helix of the γ subunit (residues Arg9 and Arg10). These electrostatic forces pull the γ shaft out of the rotational center and impede rotation through steric interference with the βE subunit. Replacement of Arg9 and Arg10 with glutamines eliminates the salt bridges and results in an activation of ATP hydrolysis activity, suggesting that these salt bridges prevent the native enzyme from rotating in ATP hydrolysis direction. A similar bending of the γ shaft as in the TA2F1 structure was observed by single-particle analysis of the TA2F1Fo holoenzyme.