In the final step of immunoproteasome assembly, the PA28-alpha-beta regulatory particle binds to the 20S immunoproteasome core particle (20S iCP) through interaction between PSME1 and PSME2 subunits of the PA28-alpha-beta heteroheptamer and the alpha (PSMA) subunits of the outer ring of the 20S iCP (Chen et al. 2021: the human PA28-alpha-beta heteroheptamer and bovine 20S iCP were used for in vitro assembly of the chimeric immunoproteasome and the subsequent cryogenic electron microscopy-based analysis - cryo-EM). Like the 26S proteasome, the regulatory particle can associate with 20S iCP on either one or both ends (Chen et al. 2021).