Inhibits proteasome 26S assembly and activity by impairing the association of the 19S regulatory complex with the 20S core. Protects SUPT6H from proteasomal degradation (By similarity). Interacts with PSMC1, PSMC2, PSMC3, PSMC4, PSMC5 and PSMC6. Interacts with SUPT6H. Belongs to the WD repeat PAAF1/RPN14 family.