PSMB5 belongs to the proteasome B-type family that has a 20S core beta subunit in the proteasome. This catalytic subunit is not present in the immunoproteasome and is substituted by catalytic subunit 3i（proteasome beta 8 subunit). A fundamental function of a modified proteasome， the immunoproteasome， is the processing of class I MHC peptides. PSMB5 exhibits an ATP dependent proteolytic activity and is involved in an ATP/ubiquitin dependent non lysosomal proteolytic pathway.
