Based on a structural study of proteasomes isolated from human embryonic kidney cell line HEK293T, the PA28-alpha-beta heteroheptamer, composed of PSME1 (PA28alpha) and PSME2 (PA28beta) subunits, forms a complex with the 20S core particle (20S CP), resulting in the PA28-alpha-beta-20S proteasome (Zhao et al. 2022). In this study, the density map of the PA28-alpha-beta-20S proteasome indicated that the PA28-alpha-beta heteroheptamer was composed of three PSME1 and four PSME2 subunits (Zhao et al. 2022), which is different from the two previously published structures of PA28-alpha-beta heteroheptamer in human (Chen et al. 2021) and mouse (Huber and Groll 2017) obtained by cryogenic electron microscopy (cryo-EM) analysis of immunoproteasomes. The function of the PA28-alpha-beta-20S proteasome is unknown but may be involved in the quick removal of damaged and unfolded proteins from the cell in response to oxidative stress (Zhao et al. 2022).