Chaperone protein which promotes assembly of the 20S proteasome. May cooperate with PSMG1-PSMG2 heterodimers to orchestrate the correct assembly of proteasomes. Homodimer. Interacts directly with alpha and beta subunits of the 20S proteasome but dissociates before the formation of half-proteasomes, probably upon recruitment of POMP (By similarity). Interacts with PSMG4. Belongs to the PSMG3 family.