The "second stalk" of Escherichia coli ATP synthase:: structure of the isolated dimerization domain

The b subunit of E. coli F0F1-ATPase links the peripheral F-1 subunits to the membrane-integral F-0 portion and functions as a "stator", preventing rotation of F-1. The b subunit is present as a dimer in ATP synthase, and residues 62-122 are required to mediate dimerization. To understand how the b subunit dimer is formed, we have studied the structure of the isolated dimerization domain, b(62-122). Analytical ultracentrifugation and solution small-angle X-ray scattering (SAXS) indicate that the b(62-122) dimer is extremely elongated, with a frictional ratio of 1.60, a maximal dimension of 95 Angstrom, and a radius of gyration of 27 A, values that are consistent with an a-helical coiled-coil structure. The crystal structure of b(62-122) has been solved and refined to 1.55 Angstrom. The protein crystallized as an isolated, monomeric cc helix with a length of 90 A. Combining the crystal structure of monomeric b(62-122) with SAXS data from the dimer in solution, we have constructed a model for the b(62-122) dimer in which the two helices form a coiled coil with a right-handed superhelical twist. Analysis of b sequences from E coli and other prokaryotes indicates conservation of an undecad repeat, which is characteristic of a right-handed coiled coil and consistent with our structural model. Mutation of residue Arg-83, which interrupts the undecad pattern, to alanine markedly stabilized the dimer, as expected for the proposed two-stranded, right-handed coiled-coil structure.