EXPERIMENTAL FIGURE 12-33 The γ subunit of the F₁ subcomplex rotates relative to the (αβ)₃ hexamer. F₁ subcomplexes were engineered to contain β subunits with an additional His-6 sequence, which causes them to adhere to a glass plate coated with a metal reagent that binds polyhistidine. The γ subunit in the engineered F₁ subcomplexes was linked covalently to a fluorescently labeled actin filament. When viewed in a fluorescence microscope, the actin filament was seen to rotate counterclockwise in discrete 120° steps in the presence of ATP due to ATP hydrolysis by the β subunits. See H. Noji et al., 1997, Nature 386:299, and R. Yasuda et al., 1998, Cell 93:1117.