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The antigenic determinants of mAbs against subunit c of the Escherichia coli ATP synthase were mapped by ELISA using overlapping synthetic heptapeptides. All epitopes recognized are located in the hydrophilic loop region and are as follows: 31-LGGKFLE-37, 35-FLEGAAR-41, 36-LEGAAR-41 and 36-LEGAARQ-42. Binding studies with membrane vesicles of different orientation revealed that all mAbs bind to everted membrane vesicles independent of the presence or absence of the F1 part. Although the hydrophilic region of subunit c and particularly the highly conserved residues A40, R41, Q42 and P43 are known to interact with subunits gamma and epsilon of the F1 part, the mAb molecules have no effect on the function of F0. Furthermore, it could be demonstrated that the F1 part and the mAb molecule(s) are bound simultaneously to the F0 complex suggesting that not all c subunits are involved in F1 interaction. From the results obtained, it can be concluded that this interaction is fixed, which means that subunits gamma and epsilon do not switch between the c subunits during catalysis and furthermore, a complete rotation of the subunit c oligomer modified with mAb(s) along the stator of the F1F0 complex, proposed to be composed of at least subunits b and delta, seems to be unlikely.

The antigenic determinants of mAbs against subunit a of the Escherichia coli ATP synthase were mapped by ELISA using overlapping synthetic decapeptides. For two of the mAbs the epitopes are E4NMTPQD10 (GDH 14-5C6) and V29DPQ32 (GDH 8-8B3). Binding of these mAbs to membrane vesicles of different orientation revealed that both epitopes are accessible in vesicles with inside-out orientation. These results demonstrate that at least the N-terminal amino acids 1-32 of subunit a are located at the cytoplasmic side of the membrane. A further determination of the topology of subunit a was performed by inserting the reporter epitope DYKDDDDK (FLAG epitope) at different positions of the polypeptide chain. 10 of 13 insertions led to a functional F0F1 ATP synthase and allowed specific detection of the modified subunit a by immunoblotting using an mAb against the FLAG epitope. In addition, polyclonal anti-FLAG IgG was applied for the recognition of the mutant FLAG epitope DYKDDVDK. Cells carrying this mutant FLAG epitope at the C terminus of subunit a were able to grow on succinate as sole carbon and energy source, revealing a functional ATP synthase, in contrast to those carrying the original FLAG epitope at the same position. Binding studies with membrane vesicles of different orientation and anti-FLAG Ig demonstrated that both termini of the protein are located at the cytoplasmic side of the membrane, indicating that an even number of membrane-spanning segments is present in subunit a. In addition, insertion of two FLAG epitopes in tandem after K66, or one epitope after H95, and Q181 revealed that the polypeptide regions including these residues are accessible from the cytoplasmic surface of the membrane. These results support the view that the polypeptide chain of subunit a traverses the membrane six times.

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.