Chaperone protein which promotes assembly of the 20S proteasome as part of a heterodimer with PSMG2. The PSMG1-PSMG2 heterodimer binds to the PSMA5 and PSMA7 proteasome subunits, promotes assembly of the proteasome alpha subunits into the heteroheptameric alpha ring and prevents alpha ring dimerization (By similarity). Forms a heterodimer with PSMG2. The PSMG1-PSMG2 heterodimer interacts directly with the PSMA5 and PSMA7 proteasome alpha subunits (By similarity). Highly expressed in testis with moderate expression in brain, liver and kidney and low levels in heart, skeletal muscle and pancreas. Expressed at a fairly constant level throughout embryonic development. Degraded by the proteasome upon completion of 20S proteasome maturation. Belongs to the PSMG1 family.