AAA+ proteins carry out diverse functions in cells. In most cases, their ATPase activity is tightly regulated by protein partners and target ligands, but the mechanism for this control has remained unclear. We have identified a conserved link between the ligand binding and ATPase sites in AAA+ proteins. This link, which we call the 'glutamate switch', regulates ATPase activity directly in response to the binding of target ligands by controlling the orientation of the conserved glutamate residue in the DExx motif, switching it between active and inactive conformations. The reasons for this level of control of the ATPase activity are discussed in the context of the biological processes catalyzed by AAA+ proteins.