ATP synthases are the smallest rotary motors in biology. The flow
of protons propels the rotation of a transmembrane entity composed of identical protein subunits III. The number of subunits in
this proton turbine determines the H+/ATP stoichiometry and
therefore the efficiency of energy conversion. In all current models
of the structure-function relationship of ATP synthases, this
rotating oligomer consists of 12 subunits. In chloroplast ATP
synthase, however, by atomic force microscopy we visualize 14
subunits III arranged in a cylindrical ring, surrounding subunit IV.
The central position of subunit IV requires redesign of the
proposed functional models of FoFrATP synthases.
The narrow and the wider orifice of the subunit III oligomer exhibit
outer diameters of 5.9 + 0.3 nm and 7.4 + 0.3 nm, respectively.
Both orifices have inner diameters of 3.5 + 0.3 nm. The
7.3 :t: 0.3 nm long oligomer traverses the 4.1 + 0.2 nm thick lipid
bilayer. In most cases 14 subunits per oligomer can directly be
counted from the images recorded. From angular power spectra
calculated from 320 individual images of well preserved particles,
the 14-fold symmetry of the proton turbine is confirmed.
Additional information is gained from the projected structure of
subcomplexes obtained by electron crystallography of 2D crystals.