PAC1-PAC2 proteasome assembly chaperone retains the core α4-α7 assembly intermediates in the cytoplasm

The proteasome core particle (CP) is a cytoplasmic and nuclear protease complex and is comprised of two α-rings and two β-rings stacked in order of αββα. The assembly of CP proceeds by ordered recruitment of β-subunits on an α-ring with help of assembly chaperones PAC1-PAC2, PAC3-PAC4, and UMP1. However, the mechanism of α-ring formation remains unsolved. Here, we show that α4, α5, α6, and α7 form a core intermediate as the initial process of α-ring assembly, which requires PAC3-PAC4. α1 and α3 can be incorporated independently into the core α4-α7 intermediate, whereas α2 incorporation is dependent on preceding incorporation of α1. Through these processes, PAC1-PAC2 prevents nonproductive dimerization of α-ring assembly intermediates. We also found that PAC1-PAC2 overrides the effect of nuclear localization signals of α-subunits and retains α-ring assembly intermediates in the cytoplasm. Our results first show a detailed assembly pathway of proteasomal α-ring and explain the mechanism by which CP assembly occurs in the cytoplasm.