We are pursuing two routes towards establishing the overall structure of the ATP synthase. The first route, where much progress has been made, is to solve high resolution structures of subcomplexes of the intact enzyme, and then to build up a mosaic structure from the individual sub-structures. The main methodology is X-ray crystallography, but sub-structures solved by solution nmr have also proved to be invaluable for establishing some details.

The substructures that have been solved so far include:

• the F₁ domain
• the F₁ domain with the bound inhibitor protein IF₁
• the F₁ domain with the attached c-ring
• a sub-complex representing most of the peripheral stalk
• a structure of the N-terminal domain of the OSCP (a peripheral stalk component) with the N-terminal region of an α-subunit

These substructures have been built into an overall low resolution structure (Figure 1, left) determined by electron microscopy of single particles of ATP synthase embedded in vitreous ice, as shown in Figure 2 (right).

The unoccupied area of the mesh in Figure 2 (right), where no high resolution structures have yet been solved, contains subunit a (or ATPase-6), which is essential for passage of protons through the membrane and generation of rotation, and four small subunits (e, f, g and A6L) which probably do not participate directly in ATP synthesis.

The second approach, which we are pursuing actively at present, is to crystallize and solve the structure of the intact ATP synthase from bovine and yeast mitochondria.