Topography of the subunits of Micrococcus lysodeikticus F₁-ATPase

Summary

The combined use of proteolytic digestion and lactoperoxidase catalyzed labelling with [¹²⁵I] applied to membrane-bound or soluble pure F₁-ATPase from Micrococcus lysodeikticus has allowed us to establish the topography of its α, β, γ and δ subunits within the protein molecule and with respect to the plane of the membrane.

The β subunit is most externally located to the membrane bilayer looking towards the cytoplasmic face, a position consistent with its proposed catalytic role. The α and γ subunits lie in an intermediate layer between the β subunits and the membrane, in which the γ subunit occupies a central position within the F₁-ATPase molecule in contact with the α subunit. The δ subunit appears to be tightly bound to the F₀ component of the ATPase complex, probably buried in the membrane bilayer. A molecular arrangement of M. lysodeikticus ATPase is proposed that, taking into account the subunit stoichiometry α₃ β₃ γ₂ δ₂ (MW 420 000), accommodates the role assigned to each subunit and most, if not all, the known properties of this bacterial energy-transducing protein.