The “Second Stalk” of Escherichia coli ATP Synthase: Structure of the Isolated Dimerization Domain,

Publisher Website
Google Scholar
Abstract
The b subunit of E. coli F0F1-ATPase links the peripheral F1 subunits to the membrane-integral F0 portion and functions as a “stator”, preventing rotation of F1. 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, b62-122. Analytical ultracentrifugation and solution small-angle X-ray scattering (SAXS) indicate that the b62-122 dimer is extremely elongated, with a frictional ratio of 1.60, a maximal dimension of 95 Å, and a radius of gyration of 27 Å, values that are consistent with an α-helical coiled-coil structure. The crystal structure of b62-122 has been solved and refined to 1.55 Å. The protein crystallized as an isolated, monomeric α helix with a length of 90 Å. Combining the crystal structure of monomeric b62-122 with SAXS data from the dimer in solution, we have constructed a model for the b62-122 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.