Acts as a chaperone during the assembly of the 26S proteasome, specifically of the base subcomplex of the 19S regulatory complex (RC). During the base subcomplex assembly is part of a NAS2:RPT4:RPT5 module; NAS2 is released during the further base assembly process. Part of a transient complex containing NAS2, RPT4 and RPT5 formed during the assembly of the 26S proteasome. Present with 846 molecules/cell in log phase SD medium. Belongs to the proteasome subunit p27 family.