ATP synthase is a two-component nanomotor. One part of the enzyme (F0) lies within the lipid bilayer and translocates protons across the membrane, whereas the other (F1) makes or breaks ATP. Recent studies have shown that each portion contains a subunit that turns within the rest of the protein framework, thus giving ATP synthase a reputation as a rotor. F1 rotation had been best shown during ATP hydrolysis, because the microscopy methods used needed soluble protein, but the enzyme requires a proton gradient across a membrane to make ATP.
To solve this problem, the German group used fluorescence resonance energy transfer to study the protein within liposomes, thus allowing them to create a proton gradient. In ATP synthesis mode, the enzyme adopted three sequential positions—similar to the 120° steps seen during hydrolysis with microscopy methods. However, the direction of rotation was opposite to that of hydrolysis. F0 is thought to rotate smoothly during proton translocation, so researchers next need to determine how that is translated to discrete steps in F1. Börsch also wonders how cellular conditions toggle the switch from synthesis to hydrolysis and back again. ▪