Low resolution structure of subunit b (b _22-156) of Escherichia coli F₁F_O ATP synthase in solution and the b-δ assembly

The first low resolution solution structure of the soluble domain of subunit b (b _22-156) of the Escherichia coli F₁F _O ATPsynthase was determined from small-angle X-ray scattering data. The dimeric protein has a boomerang-like shape with a total length of 16.2±0.3 nm. Fluorescence correlation spectroscopy (FCS) shows that the protein binds effectively to the subunit δ, confirming their described neighborhood. Using the recombinant C-terminal domain (δ_91-177) of subunit δ and the C-terminal peptides of subunit b, b _120-140 and b _140-156, FCS titration experiments were performed to assign the segments involved in δ-b assembly. These data identify the very C-terminal tail b _140-156 to interact with δ_91-177. The novel 3D structure of this peptide has been determined by NMR spectroscopy. The molecule adopts a stable helix formation in solution with a flexible tail between amino acid 140 to 145.