Structural features of the γ subunit of the Escherichia coli F₁ ATPase revealed by a 4.4-Å resolution map obtained by x-ray crystallography

The F₁ part of the F₁F₀ ATP synthase from Escherichia coli has been crystallized and its structure determined to 4.4-Å resolution by using molecular replacement based on the structure of the beef-heart mitochondrial enzyme. The bacterial F₁ consists of five subunits with stoichiometry α₃, β₃, γ, δ, and ε. δ was removed before crystallization. In agreement with the structure of the beef-heart mitochondrial enzyme, although not that from rat liver, the present study suggests that the α and β subunits are arranged in a hexagonal barrel but depart from exact 3-fold symmetry. In the structures of both beef heart and rat-liver mitochondrial F₁, less than half of the structure of the γ subunit was seen because of presumed disorder in the crystals. The present electron-density map includes a number of rod-shaped features which appear to correspond to additional α-helical regions within the γ subunit. These suggest that the γ subunit traverses the full length of the stalk that links the F₁ and F₀ parts and makes significant contacts with the c subunit ring of F₀.