THE STRUCTURE OF ATP SYNTHASE FROM CHLOROPLASTS - CONFORMATIONAL-CHANGES OF CF1 STUDIED BY ELECTRON-MICROSCOPY

The structure of the hydrophilic part of the ATP synthase from chloroplasts (CF1) has been studied by electron microscopy and image analysis of negatively stained samples. The projections of different types of CF1 were analyzed: the holoenzyme with five different subunits and CF1 depleted of either delta or epsilon and of both delta and epsilon. We studied 11 sets of 1000 projections of oxidized and reduced forms of these particles. Comparisons between the 11 average images of the various types of CF1 were made by measuring the distances between the mass centers of the alpha,beta and gamma-subunits. In the holoenzyme projections the three alpha-alpha-distances are on the average 0.9 nm larger than the beta-beta-distances. This discrepancy is caused by the presence of the small subunits in the lower half of CF1, which is close to the membrane and becomes smaller when delta is removed. The differences between the alpha-alpha and beta-beta distances enables the localization of all copies of the 5 different subunits in the hexagonal projections. It is concluded that subunits-delta and epsilon are close to one alpha-subunit and two beta-subunits. The stronger staining halo of the alpha subuints indicates that they are on a lower level in CF1 than the beta-s. Upon reduction of the holoenzyme with dithiothreitol the distances between the mass centers of gamma and the alpha-s and beta-s do not change significantly. This means that permanent conformational changes upon reduction in the plane of the hexagonal projection parallel to the membrane are relatively small. Although the alpha, beta and gamma-subunits do not change in their position, it is shown that epsilon is more flexible; it can be close to two different beta-subunits. The movements of epsilon are thought to play a role in the catalytic mechanism and regulation of ATP synthesis/hydrolysis.