Functional and idling rotatory motion within F1-ATPase

ATP synthase mediates proton flow through its membrane portion, F₀, which drives the synthesis of ATP in its headpiece, F₁. The F₁-portion contains a hexagonal array of three subunits α and three β encircling a central subunit γ, that in turn interacts with a smaller and with F₀. Recently we reported that the application of polarized absorption recovery after photobleaching showed the ATP-driven rotation of γ over at least two, if not three, β. Here we extend probes of such rotation aided by a new theory for assessing continuous versus stepped, Brownian versus unidirectional molecular motion. The observed relaxation of the absorption anisotropy is fully compatible with a unidirectional and stepping rotation of γ over three equidistantly spaced angular positions in the hexagon formed by the alternating subunits α and β. The results strongly support a rotational catalysis with equal participation of all three catalytic sites. In addition we report a limited rotation of γ without added nucleotides, perhaps idling and of Brownian nature, that covers only a narrow angular domain.