Observation of calcium-dependent unidirectional rotational motion in recombinant photosynthetic F1-ATPase molecules.

ATP hydrolysis and synthesis by the F(0)F(1)-ATP synthase are coupled to proton translocation across the membrane in the presence of magnesium. Calcium is known, however, to disrupt this coupling in the photosynthetic enzyme in a unique way: it does not support ATP synthesis, and CaATP hydrolysis is decoupled from any proton translocation, but the membrane does not become leaky to protons. Understanding the molecular basis of these calcium-dependent effects can shed light on the as yet unclear mechanism of coupling between proton transport and rotational catalysis. We show here, using an actin filament gamma-rotation assay, that CaATP is capable of sustaining rotational motion in a highly active hybrid photosynthetic F(1)-ATPase consisting of alpha and beta subunits from Rhodospirillum rubrum and gamma subunit from spinach chloroplasts (alpha(R)(3)beta(R)(3)gamma(C)). The rotation was found to be similar to that induced by MgATP in Escherichia coli F(1)-ATPase molecules. Our results suggest a possible long range pathway that enables the bound CaATP to induce full rotational motion of gamma but might block transmission of this rotational motion into proton translocation by the F(0) part of the ATP synthase.