Subunit b of the membrane moiety (F0) of ATP synthase (F1F0) from Escherichia coli is indispensable for H+ translocation and binding of the water-soluble F1 moiety.

The ATP synthase complex, designated F1F0, of Escherichia coli is composed of a water-soluble portion (F1; membrane-associated ATPase, EC 3.6.1.3) with ATP-hydrolyzing activity and a membrane-integrated part (F0) with H+-translocating activity. F0 is built up from three kinds of subunits (a, b, and c). We have isolated the F0 portion directly from membranes of an E. coli strain (KY 7485) that overproduces the enzyme several fold. Subunit b was extracted from purified F0 by two methods. One method included prolonged incubation of the F0 complex in the presence of trichloroacetate (2.5 M) and the separation of subunit b and an a-c complex by gel filtration. Alternatively, subunit b was extracted by deoxycholate and separated from the a-c complex by hydrophobic-interaction chromatography. Integrated into liposomes, the a-c complex exhibited neither H+ uptake nor binding of F1. However, a functional F0 complex was reconstituted by adding stoichiometric amounts of subunit b to the a-c complex.