Defining the domain of binding of F-1 subunit epsilon with the polar loop of F-0 subunit c in the Escherichia coli ATP synthase

We have previously shown that the E31C-substituted epsilon subunit of F-1 c an be cross-linked by disulfide bond formation to the Q42C-substituted c su bunit of F-0 in the Escherichia coli F1F0-ATP synthase complex (Zhang, Y., and Fillingame, R. H. (1995) J. Biol. Chem. 270, 24609-24614). The interact ions of subunits epsilon and c are thought to be central to the coupling of H+ transport through F-1 to ATP synthesis in F-1, To further define the do mains of interaction, we have introduced additional Cys into subunit epsilo n and subunit c and tested for cross-link formation following sulfhydryl ox idation, The results show that Cys, in a continuous stretch of residues 26- 33 in subunit epsilon, can be cross-linked to Cys at positions 40, 42, and 44 in the polar loop region of subunit c. The results are interpreted, and the subunit interaction is modeled using the NMR and x-ray diffraction stru ctures of the monomeric subunits together with information on the packing a rrangement of subunit c in a ring of 12 subunits, In the model, residues 26 -33 form a turn of antiparallel beta-sheet which packs between the polar lo op regions of adjacent subunit c at the cytoplasmic surface of the c(12) ol igomer.