The b and δ subunits of the Escherichia coli ATP synthase interact via residues in their C-terminal regions

An affinity resin for the F-1 sector of the Escherichia coli ATP synthase was prepared by coupling the b subunit to a solid support through a unique cysteine residue in the N-terminal leader. b(24-156), a form of b lacking the N-terminal transmembrane domain, was able to compete with the affinity resin for binding of F-1. Truncated forms of b(24-156), in which one or four residues from the C terminus mere removed, competed poorly for F-1 binding, suggesting that these residues play an important role in b-F-1 interactions. Sedimentation velocity analytical ultracentrifugation revealed that removal of these C-terminal residues from b(24-156) resulted in a disruption of its association with the purified delta subunit of the enzyme. To determine whether these residues interact directly with delta, cysteine residues were introduced at various C-terminal positions of b and modified with the heterobifunctional cross-linker benzophenone-4-maleimide, Cross-links between b and delta were obtained when the reagent was incorporated at positions 155 and 158 (two residues beyond the normal C terminus) in both the reconstituted b(24-156)-F-1 complex and the membrane-bound F1F0 complex. CNBr digestion followed by peptide sequencing showed the site of cross-linking within the 177-residue delta 6 subunit to be C-terminal to residue 148, possibly at Met-158. These results indicate that the b and delta subunits interact via their C-terminal regions and that this interaction is instrumental in the binding of the F-1 sector to the b subunit of F-0.