The rotor tip inside a bearing of a thermophilic F ₁-ATPase is dispensable for torque generation

F ₁-ATPase is an ATP-driven rotary molecular motor in which the central γ-subunit rotates inside a stator cylinder made of α ₃β ₃ subunits. To elucidate the role of rotor-stator interactions in torque generation, we truncated the γ-subunit at its carboxyl terminus, which forms an a helix that penetrates deeply into the stator cylinder. We used an α ₃β ₃γ subcomplex of F ₁-ATPase derived from thermophilic Bacillus PS3 and expressed it in Escherichia coli. We could obtain purified subcomplexes in which 14, 17, or 21 amino-acid residues were deleted. The rotary characteristics of the truncated mutants, monitored by attaching a duplex of 0.49-μm beads to the γ-subunit, did not differ greatly from those of the wild-type over the ATP concentrations of 20 nM-2 mM, the most conspicuous effect being ∼50% reduction in torque and ∼70% reduction in the rate of ATP binding upon deletion of 21 residues. The ATP hydrolysis activity estimated in bulk samples was more seriously affected. The 21-deletion mutant, in particular, was >10-fold less active, but this is likely due to instability of this subcomplex. For torque generation, though not for rapid catalysis, most of the rotor-stator contacts on the deeper half of the penetrating portion of the γ-subunit are dispensable.