Mutations in the dimerization domain of the b subunit from the Escherichia coli ATP synthase -: Deletions disrupt function but not enzyme assembly

The b subunit dimer of Escherichia coli ATP synthase serves essential roles as an assembly factor for the enzyme and as a stator during rotational catalysis. To investigate the functional importance of its coiled coil dimerization domain, a series of internal deletions including each individual residue between Lys-100 and Ala-105 (b(Delta K100)-b(Delta A105)), b(Delta K100-A103), and b(Delta K100-Q106) as well as a control b(K100A) missense mutation were prepared. All of the mutants supported assembly of ATP synthase, but all single-residue deletions failed to support growth on acetate, indicating a severe defect in oxidative phosphorylation, and b(Delta K100-Q106) displayed moderately reduced growth. The membrane-bound ATPase activities of these strains showed a related reduction in sensitivity to dicyclohexylcarbodiimide, indicative of uncoupling. Analysis of dimerization of the soluble constructs of b(Delta K100) and the multiple-residue deletions by sedimentation equilibrium revealed reduced dimerization compared with wild type for all deletions, with b(Delta K100-Q106) most severely affected. In cross-linking studies it was found that F-1-ATPase can mediate the dimerization of some soluble b constructs but did not mediate dimerization of b(Delta K100) and b(Delta K100-Q106); these two forms also were defective in F-1 binding analyses. We conclude that defective dimerization of soluble b constructs severely affects F-1 binding in vitro, yet allows assembly of ATP synthase in vivo. The highly uncoupled nature of enzymes with single-residue deletions in b indicates that the b subunit serves an active function in energy coupling rather than just holding on to the F-1 sector. This function is proposed to depend on proper, specific interactions between the b subunits and F-1.