Acts as a chaperone during the assembly of the 26S proteasome, specifically of the base subcomplex of the PA700/19S regulatory complex (RC). In the initial step of the base subcomplex assembly is part of an intermediate PSMD5:PSMC2:PSMC1:PSMD2 module which probably assembles with a PSMD10:PSMC4:PSMC5:PAAF1 module followed by dissociation of PSMD5 (By similarity). Interacts with PSMC1, PSMC2, PSMD1 and PSMD6. Part of transient complex containing PSMD5, PSMC2, PSMC1 and PSMD2 formed during the assembly of the 26S proteasome (By similarity). Rich in dileucine repeats, which have been implicated in trafficking of a variety of transmembrane proteins. Belongs to the proteasome subunit S5B/HSM3 family.