The b subunit of Escherichia coli ATP synthase.

The b subunit of ATP synthase is a major component of the second stalk connecting the F1 and F0 sectors of the enzyme and is essential for normal assembly and function. The 156-residue b subunit of the Escherichia coli ATP synthase has been investigated extensively through mutagenesis, deletion analysis, and biophysical characterization. The two copies of b exist as a highly extended, helical dimer extending from the membrane to near the top of F1, where they interact with the delta subunit. The sequence has been divided into four domains: the N-terminal membrane-spanning domain, the tether domain, the dimerization domain, and the C-terminal delta-binding domain. The dimerization domain, contained within residues 60-122, has many properties of a coiled-coil, while the delta-binding domain is more globular. Sites of crosslinking between b and the a, alpha, beta, and delta subunits of ATP synthase have been identified, and the functional significance of these interactions is under investigation. The b dimer may serve as an elastic element during rotational catalysis in the enzyme, but also directly influences the catalytic sites, suggesting a more active role in coupling.