Structural insights on ATP hydrolysis-driven mechanical work of AAA+ hexamers

N-ethylmaleimide-sensitive factor (NSF) and ClpX are homo-hexameric proteins of the AAA+ (ATPases Associated with diverse cellular Activities) family. Using ATP, NSF recycles SNARE complexes following membrane fusion, while ClpX unfolds and translocates proteins through its pore. However, their molecular mechanisms were unclear until recently. NSF efficiently disassembles a SNARE complex using ATP that were bound before SNARE binding, by changing from a ‘split washer’ to a ‘flat washer’ conformation. ClpX utilizes numerous ATP binding and hydrolyses for translocation. Structural studies of ClpX show that two of the six ATP sites are unloadable. Hence, in ClpX, it is believed ATP hydrolyses occur in pairs and in symmetric motifs to work. Overall, NSF follows a spring loaded model, while ClpX follows a power-stroke model – showing that even for proteins that belong to the same family and that have similar structures, functions and models of action can be very different.