Plays an important role in control of proteasome function. Inhibits the hydrolysis of protein and peptide substrates by the 20S proteasome. Also inhibits the activation of the proteasome by the proteasome regulatory proteins PA700 and PA28. Monomer and homodimer. Interacts with FBXO7. Interacts with the 20S proteasome. Belongs to the proteasome inhibitor PI31 family.