Stairway to translocation: AAA+ motor structures reveal the mechanisms of ATP-dependent substrate translocation

Translocases of the AAA+ (ATPases Associated with various cellular Activities) family are powerful molecular machines that use the mechano-chemical coupling of ATP hydrolysis and conformational changes to thread DNA or protein substrates through their central channel for many important biological processes. These motors comprise hexameric rings of ATPase subunits, in which highly conserved nucleotide-binding domains form active-site pockets near the subunit interfaces and aromatic pore-loop residues extend into the central channel for substrate binding and mechanical pulling. Over the past 2 years, 41 cryo-EM structures have been solved for substrate-bound AAA+ translocases that revealed spiral-staircase arrangements of pore-loop residues surrounding substrate polypeptides and indicating a conserved hand-over-hand mechanism for translocation. The subunits' vertical positions within the spiral arrangements appear to be correlated with their nucleotide states, progressing from ATP-bound at the top to ADP or apo states at the bottom. Studies describing multiple conformations for a particular motor illustrate the potential coupling between ATP-hydrolysis steps and subunit movements to propel the substrate. Experiments with double-ring, Type II AAA+ motors revealed an offset of hydrolysis steps between the two ATPase domains of individual subunits, and the upper ATPase domains lacking aromatic pore loops frequently form planar rings. This review summarizes the critical advances provided by recent studies to our structural and functional understanding of hexameric AAA+ translocases, as well as the important outstanding questions regarding the underlying mechanisms for coordinated ATP-hydrolysis and mechano-chemical coupling.