Three-Dimensional Structure of an Active form of Mithocondrial F1-ATPase. M.A.Bianchet*, J. Hullihen^[daggerdbl], P. L.Pedersen^[daggerdbl] & L. M. Amzel*. *Dept. of Biophysics & Biophysical Chemistry, ^[daggerdbl]Dept. of Biological Chemistry. Johns Hopkins Medical School. Baltimore, MD 21205

F1-ATPase is the soluble part of the F1F0-ATP synthase complex, the protein that carries out the synthesis of ATP in the mitochondria. The F1 portion exhibits a Mg⁺⁺ dependent ATP hydrolytic activity. Crystals of rat liver F1-ATPase were obtained using (NH4)2SO4 as a precipitant in the presence of ATP, Pi and in the absence of Mg. Under the crystallization condition used, rat liver F1-ATPase is catalytic active. F1 from redissolved crystals has all subunits and hydrolyzes ATP with full activity when Mg⁺⁺ is added. The 3-dimensional structure of the rat liver F1-ATPase determined to 2.8 Å resolution is presented here. In the rat liver F1-crystals the three a/[beta] pairs are all in the same conformations as expected from the crystal symmetry. Its cell dimensions and symmetry indicate that its asymmetric unit contains a third of the F1 oligomer (a3[beta]3[gamma]d[epsilon]). The small subunits d, [epsilon] are not observed, because they can not comply with such symmetry, suggesting that, they do not participate in crystal contacts and adopt different positions in each lattice point. The observation of three copies of the [gamma]-subunit related by the three fold axis confirms this point. The nucleotides bound to both a and [beta] subunits have been identified using occupancy refinement scheme. The more symmetrical conformation of the a and [beta] subunits observed in the rat liver F1 vis-a-vis bovine heart suggests that it may be in another stage of the proposed mechanism or perhaps suggests a different one.