The proteasome holoenzyme regulates the cellular proteome via degrading most proteins. In its 19-subunit regulatory particle (RP), a heterohexameric ATPase enables protein degradation by injecting protein substrates into the core peptidase. RP assembly utilizes "checkpoints," where multiple dedicated chaperones bind to specific ATPase subunits and control the addition of other subunits. Here, we find that the RP assembly checkpoint relies on two common features of the chaperones. Individual chaperones can distinguish an RP, in which their cognate ATPase persists in the ATP-bound state. Chaperones then together modulate ATPase activity to facilitate RP subunit rearrangements for switching to an active, substrate-processing state in the resulting proteasome holoenzyme. Thus, chaperones may sense ATP binding and hydrolysis as a readout for the quality of the RP complex to generate a functional proteasome holoenzyme. Our findings provide a basis to potentially exploit the assembly checkpoints in situations with known deregulation of proteasomal ATPase chaperones.