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. Forms a heterodimer with PSMG1. The PSMG1-PSMG2 heterodimer interacts directly with the PSMA5 and PSMA7 proteasome alpha subunits. Widely expressed with highest levels in lung, brain and colon. Moderately expressed in muscle, stomach, spleen and heart. Weakly expressed in small intestine, pancreas and liver. Highly expressed in hepatocellular carcinomas with low levels in surrounding liver tissue. Degraded by the proteasome upon completion of 20S proteasome maturation. The disease may be caused by variants affecting the gene represented in this entry. Belongs to the PSMG2 family. Truncated N-terminus.