The crystallization of beef heart mitochondrial adenosine triphosphatase

Electron-microscopic studies on location of SH-groups in mitochondrial F1-ATPase using a ferritin label

A new approach has been suggested for electron-microscopic study of the structure of mitochondrial F1-ATPase based on ferritin labeling. By means of sequential treatment with 2-iminothiolane and Nbs2 we obtained a modified ferritin (NbsSPrCNH-Ft) able to react with SH-groups of proteins and to form conjugates in which the protein and ferritin are bound by disulfide bonds. An electron-microscopic investigation of the negatively stained preparations of mitochondrial F1-ATPase, preincubated with modified ferritin, revealed such enzyme-ferritin conjugates. In case of modified ferritin, containing 360 mol SH-groups per mol protein, and F1-ATPase, pretreated with N-ethylmaleimide and then with dithiothreitol, conjugates were obtained in which ferritin molecules are bound to several (as many as four) of the six protein masses, comprising a bilayer molecule of the enzyme. Taking into consideration the biochemical data on the location of accessible SH-groups (only in alpha, gamma or epsilon subunits), it is inferred from the results obtained that one of the protein masses is a complex between beta subunit and at least one of the minor subunits located partially on the molecule's external side. This indicates the nonequivalence of different copies of the major subunits. Averaged images of the particles of the F1-F0 complex from bovine heart mitochondria and bacteria Micrococcus lysodeicticus were obtained. It was found that F0 component is bound to two adjacent protein masses of the F1-ATPase molecule. It is suggested that this binding may be due the nonequivalency of single-type major subunits.

An electron microscopic approach to the quaternary structure of mitochondrial F1-ATPase

The three-dimensional structure of F1-ATPase from beef heart mitochondria was investigated by electron microscopic techniques. The presence of high concentrations of nucleotides is essential for preservation of the quaternary structure. When investigated under such conditions, monodisperse F1-ATPase could not be distinguished from the membrane-bound enzyme. At low resolution, the particle shape resembles an oblate ellipsoid of revolution with an axial ratio of about 2:1. From several lines of evidence (including field micrographs at higher magnifications, Markham rotational analysis, and tilting experiments), two conclusions may be drawn concerning the three-dimensional fine structure of F1-ATPase. 1. At the periphery of the molecule, six globular protein masses are orientated in a way similar to the chair conformation of cyclohexane. This array is interpreted to be made up of an alternating sequence of alpha and beta subunits. 2. Part of the central space is occupied by a seventh protein mass, protrusions of which are likely to be in contact with some of the outer subunits. A gamma subunit is supposed to be constituent part of this central protein mass. As a consequence, this model favours a stoichiometry of alpha 3 beta 3 gamma for the large subunits of beef heart F1-ATPase.

Structure and function of H+-ATPase

(1) Extensive studies on proton-translocating ATPase (H+-ATPase) revealed that H+-ATPase is an energy transforming device universally distributed in membranes of almost all kinds of cells. (2) Crystallization of the catalytic portion (F1) of H+-ATPase showed that F1 is a hexagonal molecule with a central hole. The diameter of F1 is about 90 A and its molecular weight is about 380,000. (3) Use of thermophilic F1 permits the complete reconstitution of F1 from its five subunits (alpha, beta, gamma, delta, epsilon) and demonstration of the gate function of the gamma delta epsilon-complex, the catalytic function of beta (supported by alpha and gamma), and the H+-translocating functions of all five subunits. (4) Studies using purified thermostable F0 showed that F0 is an H+-channel portion of H+-ATPase. The direct measurement of H+-flux through F0, sequencing of DCCD-binding protein, and isolation of F1-binding protein are described. (5) The subunit stoichiometry of F1 may be alpha 3 beta 3 gamma delta epsilon. (6) Reconstitution of stable H+-ATPase-liposomes revealed that ATP is directly synthesized by the flow of H+ driven by an electrochemical potential gradient and that H+ is translocated by ATP hydrolysis. This rules out functions for all the hypothetical components that do not belong to H+-ATPase in H+-driven ATP synthesis. The roles of conformation change and other phenomena in ATP synthesis are also discussed.