FIGURE 3.

Proposed models of the rotation mechanism of Enterococcus hirae V₁-ATPase based on the crystal structures. [Model 1 (Suzuki et al., 2016)] Upper drawings show the structural models from the left to the right panel based on the crystal structures of 2_ATPV₁ (catalytic dwell), 2_ADPV₁ (ATP-binding dwell), 3_ADPV₁ (ADP-release dwell), and 2_ATPV₁. ATP indicated with a yellow ‘P’ in 2_ATPV₁ represents an ATP molecule that is committed to hydrolysis. Lower drawing shows the coupling model of the 120° rotation of the shaft (green ellipse with white arrow) and the ATP hydrolysis based on the structural model (upper drawings). Each circle represents the conformation of the nucleotide-binding site, viewed from the cytoplasmic side. The orientation of the shaft begins from the 12 o’clock position in the catalytic dwell waiting for ATP hydrolysis. ATP^∗ represents an ATP molecule that is committed to hydrolysis. ATP^∗ is hydrolyzed to produce ADP and Pi, and the Pi release induces the conformational changes to the ATP-binding dwell state, without a rotational sub-step. ATP binding at the ‘bindable-like’ form in the ATP-binding dwell state, induces the conformational changes to the ADP-release dwell, without an apparent rotational sub-step. ADP release from the ‘tight-like’ form induces the dissociation of the shaft, thermal 120° rotation, and consequent conformational changes to the catalytic dwell. [Model 2] An alternative coupling scheme of the model 1 without the ADP-release dwell state is shown. ATP binding to the ATP-binding dwell state induces the concomitant release of the shaft and ADP. Therefore, this transient intermediate structure may correspond to that of 2_ATPA₃B₃ with the shaft (green circle) thermally fluctuating. For details, see text.