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 in vistigated under such coinditions, 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‐dimenwsional 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 interpretated to be made up of an alternating sequence of α and β 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 γ subunit is supposed to be constituent part of this central protein mass. As a consequence, this model favours a stoichiometry oif