PSMD9 is a multicatalytic proteinase complex with a highly ordered structure composed of 2 complexes, a 20S core and a 19S regulator. PSMD9 acts as a chaperone through the assembly of the 26S proteasome, specifically of the base subcomplex of the PA700/19S regulatory complex(RC). During the base subcomplex compilation, PSMD9 is part of an intermediate PSMD9:PSMC6:PSMC3 module, also called modulator trimer complex. PSMD9 is released during the further base assembly process.