Kühlbrandt is interested in the structure of membrane proteins, which are normally extracted from their surroundings before examination. But with the recent emergence of electron tomography, Kühlbrandt realized, “if a protein is large enough, it should be possible to visualize it in situ without taking the whole thing apart.” His team tried its luck on ATP synthase embedded in bits of mitochondrial membrane.
The images revealed long ribbons of ATP synthase dimers in highly curved membranes. Previous work suggests that the curvature is created by the dimers: yeast mutants lacking dimerization subunits are missing the elaborate mitochondrial membrane invaginations called cristae. And the massive synthase leaves little room for anything else to do the bending.
The curves, the group suggests, make the synthase more efficient by increasing the local pH gradient, which helps drive ATP synthesis. The gradient is created by the pumping of protons into the inner membrane space. Mathematical calculations revealed that those protons can be more densely packed along membrane curves, thereby increasing the pH gradient there, where ATP synthase is situated.
Bacteria get by with only monomeric ATP synthase in flat membranes. But eukaryotes probably evolved dimers rapidly, as they are found in yeast, unicellular ciliates, and mammals.