The role of the stalk in the coupling mechanism of F1F0‐ATPases

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Abstract
The extrinsic and intrinsic membrane sectors of F1F0-ATPases are linked by a slender stalk 40–50 Å in length. The stalk transmits the energy produced by oxidative or photosynthetic phosphorylation from the intrinsic sector, F0, to the catalytic sites in the extrinsic F1 sector. How this is achieved is unknown, but long-range conformational changes linked to transmembrane proton transport may be involved. In bacterial and chloroplast F1F0-ATPases, the stalk is probably a composite of subunits δ and ε, part of the γ-subunit, and the extrinsic membrane domains of 2 subunits (identical or non-identical according to the species) that are bound to the membrane by their N-terminal regions. The stalk in the bovine mitochondrial enzyme appears to be more complex, and the γ, δ, ε, OSCP, F6, b and d subunits all contribute to it. A bovine stalk complex has been assembled in vitro from bacterially expressed OSCP, F6, b and d, both in the presence and in the absence of F1-ATPase. One molecule of each of these subunits is present in the assembled complexes, as there is also in each native F1F0-ATPase assembly. Providing that suitable crystals can be obtained, the stalk complex and the F1·stalk complex may permit the high resolution structure of bovine F1-ATPase to be extended into the stalk domain.