Characterization of a b2δ complex from Escherichia coli ATP synthase

Abstract

The δ subunit of Escherichia coli ATP synthase has been expressed and purified, both as the intact polypeptide and as δ', a proteolytic fragment composed of residues 1-134. The solution structure of δ' as a five-helix bundle has been previously reported (Wilkens, S., Dunn, S. D., Chandler, J., Dahlquist, F. W., and Capaldi, R. A. (1997) Nat. Struct. Biol. 4, 198-201). The δ subunit, in conjunction with δ-depleted F1-ATPase, was fully capable of reconstituting energy-dependent fluorescence quenching in membrane vesicles that had been depleted of F1. A complex of δ with the cytoplasmic domain of the b subunit of F0 was demonstrated and characterized by analytical ultracentrifugation using b(ST34-156), a form of the b domain lacking aromatic residues. Molecular weight determination by sedimentation equilibrium supported a b2δ subunit stoichiometry. The sedimentation coefficient of the complex, 2.1 S, indicated a frictional ratio of approximately 2, suggesting that δ and the b dimer are arranged in an end- to-end rather than side-by-side manner. These results indicate the feasibility of the b2δ complex reaching from the membrane to the membrane- distal portion of the F1 sector, as required if it is to serve as a second stalk.