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 α3β3 complex has a hexagonal, apparently symmetrical arrangement of masses with a central cavity. (2) The projections of the α3β3δ complexes are similar to those of the α3β3 complexes. (3) In contrast, the α3β3λ complex has an additional mass in the centre which is similar to that found in the native enzyme (α3β3γδϵ). From these observations, it is concluded that the central mass in the F1-ATPase is comprised mostly of the γ subunit.
