Stabilization of Fo/Vo/Ao by a radial electric field

Abstract

The membrane domain of rotary ATPases (Fo/Vo/Ao) contains a membrane-embedded rotor ring which rotates against an adjacent cation channel-forming subunit during catalysis. The mechanism that allows stabilization of the highly mobile and yet tightly connected domains during operation while not impeding rotation is unknown. Remarkably, all known ATPase rotor rings are filled by lipids. In the crystal structure of the rotor ring of a V-ATPase from Enterococcus hirae the ring filling lipids form a proper membrane that is lower with respect to the embedding membrane surrounding both subunits. I propose first, that a vertical shift between lumenal lipids and embedding outside membrane is a general feature of rotor rings and second that it leads to a radial potential fall-off between rotor ring and cation channel, creating attractive forces that impact rotor-stator interaction in Fo/Vo/Ao during rotation.

Keywords: ATP synthase; electrochemical gradient; membrane protein; transmembrane electric field.

Figure 1

Schematic representation of rotary ATPases. Rotor parts are depicted in red, stator parts in blue. Cation channel forming stator and membrane embedded cation transporting rotor ring are highlighted by darker hues.

Figure 2

Central slice of the K-ring from Enterecoccus hirae (pdb 2BL2) perpendicular to the membrane plane. Protein is depicted in white ball&stick, protein surface in light grey, sodium as a red sphere, water as blue spheres and lipid as ball&stick and surface in gold-orange. Boundaries of the inside membrane and the expected position of the outside membrane are indicated by overlayed boxes. Notice the coincidence of height between outside bound sodium and inside lipid headgroups.

Figure 3

(A) Cross-section through the K-ring horizontal to the membrane plane at the height of the sodium binding sites. (B) Close-up of the same cross-section. The proximity of the bound sodium to structural water of the ring inside is indicated by broken lines and the horizontal line-up of charges by the colored spheres of a lipid head group phosphate (yellow), the terminal nitrogen of lysine 32 (green) and a structural water (blue).

Figure 4

Cartoon illustrating how horizontal proximity of the cation binding site and bulk solution on the inside of the rotor ring could result in a radial membrane potential fall-off, indicated by a purple triangle, at the interface of the rotor ring and the adjacent subunit. Regions of high dielectric constant are depicted in blue for bulk solution and red for lipid headgroups. Regions of low dielectric constant are shown in grey for protein and in yellow for the hydrophobic membrane core.