Structure of mitochondrial F1-ATPase studied by electron microscopy and image processing

The structure of soluble F1-ATPase (EC 3.6.1.3) has been investigated by computer analysis of individual molecular images extracted from electron micrographs of negatively stained particles. A total of 1241 images was interactively selected from several digitized micrographs and these images were subsequently aligned relative to different reference images. They were then submitted to a multivariate statistical classification procedure. We have focussed our attention on the main 'hexagonal' view which represents some 40 % of our population of images. In this view, six masses are located on the outer region of the projection which are associated with the alpha and the beta subunits of the protein. A seventh mass is located close to the centre of the hexagon, but slightly off its exact midpoint. It has the shape of the letter V and its two legs point to two of the outer protein masses, or one alpha-beta subunit pair. The corner of the V has a density as high as those of the large subunits. Possible subunit arrangements and their consequences for the mechanism of ATP synthesis are discussed.