ATPase-dedicated chaperone that assists the formation of the RPT6-RPT3 ATPase pair, an early step in proteasome assembly. Plays a key role in maintaining homeostatic proteasome levels and adjusting proteasome assembly when demands increase, such as during proteasome stresses. Function overlaps with RPN14. Interacts with RPT6. Interacts with the 40S and 60S ribosomal subunits. Expression is increased in absence of RPN14 and upon treatment with tunicamycin, an agent which causes accumulation of misfolded proteins in the endoplasmic reticulum. Decreases cell fitness. Causes severe growth defects and an overload of polyubiquitinated conjugates when associated with a RPT6 thermosensitive proteasome mutant. Present with 2110 molecules/cell in log phase SD medium.