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.