An Electron Microscopic Approach to the Quaternary Structure of Mitochondrial F1‐ATPase

Abstract

The 3-dimensional structure of F1-ATPase from beef heart mitochondria was investigated by EM 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 particles shape resembles an oblate ellipsoid of revolution with an axial ratio of .apprx. 2:1. From several lines of evidence (including field micrographs at higher magnifications, Markham rotational analysis and tilting experiments), 2 conclusions may be drawn concerning the 3-dimensional fine structure of F1-ATPase. At the periphery of the molecule, 6 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 .alpha. and .beta. subunits. Part of the central space is occupied by a 7 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 favors a stoichiometry of .alpha.3.beta.3.gamma. for the large subunits of beef heart F1-ATPase.