The F0 complex of the Escherichia coli ATP synthase. Investigation by electron spectroscopic imaging and immunoelectron microscopy.

Cholate-solubilized F0 complexes of the ATP synthase (F0F1) from Escherichia coli were studied by application of conventional transmission electron microscopy and electron spectroscopic imaging (ESI) of negatively stained samples. Using the ESI mode, the structural organization of the F0 complex (diameter of 7.5 +/- 0.5 nm) could be observed in more detail and defined projections could be distinguished. Projection A appears as a deltoid-like structure with bilateral symmetry. Projection B has an overall trapezoidal shape with some similarity in shape to the letter W. Applying the ESI mode to the ac complex dissolved in cholate-containing buffer, an elongated structure consisting of two intensity maxima could be observed. Simulations with models of the F0 and the ac complex revealed that the projections observed can be obtained by tilting and rotating a model in which subunit a and the two copies of subunit b are located outside the subunit c oligomer. This view of structural organization was supported by results obtained with F0 complexes decorated with monoclonal antibodies against subunits a, b or c.