Inhibits proteasome 26S assembly and proteolytic activity by impairing the association of the 19S regulatory complex with the 20S core. In case of HIV-1 infection, recruited by viral Tat to the HIV-1 promoter, where it promotes the recruitment of 19S regulatory complex through dissociation of the proteasome 26S. This presumably promotes provirus transcription efficiency. Protects SUPT6H from proteasomal degradation. Interacts with PSMC1, PSMC2, PSMC3, PSMC4, PSMC5 and PSMC6. Interacts with SUPT6H. (Microbial infection) Interacts with HIV-1 Tat. Ubiquitously expressed, with highest levels in kidney, brain and testis. Belongs to the WD repeat PAAF1/RPN14 family.