Acts as a chaperone during the assembly of the 26S proteasome, specifically of the base subcomplex of the PA700/19S regulatory complex (RC). During the base subcomplex assembly is part of an intermediate PSMD9:PSMC6:PSMC3 module, also known as modulator trimer complex; PSMD9 is released during the further base assembly process. Interacts with PSMC3. Part of a transient complex (modulator) containing PSMD9, PSMC6 and PSMC3 formed during the assembly of the 26S proteasome. Expressed in all tissues tested, highly expressed in liver and kidney. Belongs to the proteasome subunit p27 family. Was initially identified as a component of the 26S proteasome.