PSMF1 is a protein which hinders the activation of the proteasome by the 11S and 19S regulators. The 26S proteasome is a multicatalytic proteinase complex with an extremely ordered structure composed of 2 complexes, a 20S core and a 19S regulator. The 20S core is comprised of 4 rings of 28 non-identical subunits; 2 rings are composed of 7 alpha subunits and 2 rings are composed of 7 beta subunits.