F1-ATPase conformational cycle from simultaneous single-molecule FRET and rotation measurements

Despite extensive studies, the structural basis for the mechanochemical coupling in the rotary molecular motor F-1-ATPase (F-1) is still incomplete. We performed single-molecule FRET measurements to monitor conformational changes in the stator ring-alpha(3)beta(3), while simultaneously monitoring rotations of the central shaft-gamma. In the ATP waiting dwell, two of three beta-subunits simultaneously adopt low FRET nonclosed forms. By contrast, in the catalytic intermediate dwell, two beta-subunits are simultaneously in a high FRET closed form. These differences allow us to assign crystal structures directly to both major dwell states, thus resolving a long-standing issue and establishing a firm connection between F-1 structure and the rotation angle of the motor. Remarkably, a structure of F-1 in an epsilon-inhibited state is consistent with the unique FRET signature of the ATP waiting dwell, while most crystal structures capture the structure in the catalytic dwell. Principal component analysis of the available crystal structures further clarifies the five-step conformational transitions of the alpha beta-dimer in the ATPase cycle, highlighting the two dominant modes: the opening/closing motions of beta and the loosening/tightening motions at the alpha beta-interface. These results provide a new view of tripartite coupling among chemical reactions, stator conformations, and rotary angles in F-1-ATPase.