The 26S proteasome, the central eukaryotic protease, comprises a core particle capped by a 19S regulatory particle (RP). The RP is divisible into base and lid subcomplexes. Lid biogenesis and incorporation into the RP remain poorly understood. We report several lid intermediates, including the free Rpn12 subunit and a lid particle (LP) containing the remaining eight subunits, LP2. Rpn12 binds LP2 in vitro, and each requires the other for assembly into 26S proteasomes. Stable Rpn12 incorporation depends on all other lid subunits, indicating that Rpn12 distinguishes LP2 from smaller lid subcomplexes. The highly conserved C terminus of Rpn12 bridges the lid and base, mediating both stable binding to LP2 and lid-base joining. Our data suggest a hierarchical assembly mechanism where Rpn12 binds LP2 only upon correct assembly of all other lid subunits, and the Rpn12 tail then helps drive lid-base joining. Rpn12 incorporation thus links proper lid assembly to subsequent assembly steps.
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