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 (By similarity). Interacts with PSMC3. Part of a transient complex (modulator) containing PSMD9, PSMC6 and PSMC3 formed during the assembly of the 26S proteasome (By similarity). Belongs to the proteasome subunit p27 family.