Quaternary structure of V¹ and F¹ ATPase: Significance of structural homologies and diversities

The V₁ ATPase from the tobacco hornworm Manduca sexta and the Escherichia coli F₁ ATPase were characterized by small-angle X-ray scattering (SAXS). The radii of gyration (R(g)) of the complexes were 6.2 ± 0.1 and 4.7 ± 0.02 nm, respectively. The shape of the M. sexta V₁ ATPase was determined ab initio from the scattering data showing six masses, presumed to be the A and B subunits, arranged in an alternating manner about a 3-fold axis. A seventh mass with a length of about 11.0 nm extends perpendicularly to the center of the hexameric unit. This central mass is presumed to be the stalk that connects V₁ with the membrane domain (V₀) in the intact V₁V₀-ATPase. In comparison, the shape of the F₁ ATPase from E. coli possesses a quasi-3-fold symmetry over the major part of the enzyme. The overall asymmetry of the structure is given by a stem, assumed to include the central stalk subunits. The features of the V₁ and F₁ ATPase reveal structural homologies and diversities of the key components of the complexes.