Structure of Blm10:13S proteasome intermediate reveals parallel assembly pathways for the proteasome core particle.

The proteasome core particle (CP) can be capped by Blm10/PA200, which promotes ATP-independent degradation of disordered proteins and contributes to attenuating antigenic diversity and impacts immunotherapy upon upregulation. The assembly process that leads to Blm10-CP proteasomes remains unresolved and Blm10 is present not only at mature proteasomes but also bound to assembly intermediates. We identified and solved structures of five CP intermediate complexes by cryo-electron microscopy, two of which include Blm10. We find that CP intermediates interactions with Blm10 and Pba1/Pba2 chaperone complex are mutually exclusive and both leads to mature CP production. We further find no synergy between Pba1/Pba2 and Blm10 for CP assembly. These data suggest a parallel assembly pathway exists for Blm10 proteasomes that does not require sequential activity with Pba1/Pba2. Whereas CP affinity for Pba1/Pba2 is reduced during maturation to promote its release, no such affinity switch occurs for Blm10, further suggesting a parallel Blm10-assembly pathway for production of mature Blm10-bound CPs. Altogether, our findings reveal a functionally distinct CP assembly pathway for the production of mature Blm10-CP proteasomes.