Molecular chaperone essential for the assembly of standard proteasomes and immunoproteasomes. Degraded after completion of proteasome maturation. Mediates the association of 20S preproteasome with the endoplasmic reticulum. {ECO:0000269|PubMed:15944226, ECO:0000269|PubMed:16251969, ECO:0000269|PubMed:17948026}. This is the function of Ensembl gene identifier ENSG00000132963 (POMP, proteasome maturation protein).