Electron microscopy of the reconstituted complexes of the F1-ATPase with various subunit constitution revealed the location of the gamma subunit in the central cavity of the molecule.

The subunit structure of the F1-ATPase from the thermophilic bacterium PS3 was probed by comparing electron microscopic images of the subunit complexes reconstituted to contain different subunit compositions. The following structural features were observed. (1) The alpha 3 beta 3 complex has a hexagonal, apparently symmetrical arrangement of masses with a central cavity. (2) The projections of the alpha 3 beta 3 delta complexes are similar to those of the alpha 3 beta 3 complexes. (3) In contrast, the alpha 3 beta 3 gamma complex has an additional mass in the centre which is similar to that found in the native enzyme (alpha 3 beta 3 gamma delta epsilon). From these observations, it is concluded that the central mass in the F1-ATPase is comprised mostly of the gamma subunit.