Structure of the ATP synthase from chloroplasts studied by electron microscopy. Localization of the small subunits

The structure of the hydrophilic part of the ATP synthase from chloroplasts (CF1) has been further investigated by electron microscopy and image analysis of negatively stained samples. The projections of three different types of CF1 were analyzed: the holoenzyme with five different subunits and two CF1 particles depleted of δ and of δ,ε, respectively. About 1000 images of molecular projections of each of the three types were analyzed. They were aligned relative to different reference images and were then submitted to a multivariate statistical classification procedure. The analysis has been focussed on the projections of the hexagonal type. Comparisons between the average images of the three types of CF1 and between the previously analyzed CF1- and mitochondrial MF1 ATP synthases clearly outline the many detailed similarities but also the differences. Removal of δ leads to a significant decrease in the diameter of CF1: the distances between opposive large α and β subunits decrease by 1.3 nm. The additional removal of ε has no further effect on these distances. The positions of the small subunits could be localized within the α3β3 structure. Subunit γ, is located in the center of the hexagonal projection and has dimensions of 2.5-2.8 nm in the plane and about 5 nm in the vertical direction. Subunits δ and ε are located between γ and one pair of large α and β subunits, resulting in the V-shaped central mass, as found previously. The position of δ or ε is perhaps variable, since among the previously analyzed 3300 CF1 projections, many projections were found with the central mass divided in two parts, with the smallest mass in different positions. Consequences of these findings for the mechanism of ATP synthesis are discussed