Chaperone protein which promotes assembly of the 20S proteasome as part of a heterodimer with PSMG1. 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 PSMG1. The PSMG1-PSMG2 heterodimer interacts directly with the PSMA5 and PSMA7 proteasome alpha subunits (By similarity). Expression is elevated in proliferating cells and down-regulated in cells undergoing growth arrest. During the cell cycle, expression is low in G1-arrested cells, increases during S phase and remains high at G2-M phase. Degraded by the proteasome upon completion of 20S proteasome maturation. Belongs to the PSMG2 family.