A. Ump1 functions as a checkpoint protein in 20S assembly. Ump1 associates with 15S intermediates (I) in yeast (13S–16S in mammals) consisting of a full complement of α subunits and a subset of β subunits (β2, β3, β4 in yeast). Subsequent entry of β subunits leads to a half-proteasome species that contains all subunits except β7 (II). During this time, Ump1 is acting to prevent the premature dimerization (not shown) of any/all species along the assembly pathway from (I) through (II). Addition of β7 (III) represents the rate limiting step of 20S assembly, and leads to a conformational change that relieves the inhibitory function of Ump1 (denoted by the shift in position towards β6). This is quickly followed by the β5-propeptide-mediated dimerization of two half proteasomes (IV). Ump1 becomes encapsulated in the pre-holoproteasome and is degraded following the processing of the β-subunit propeptides to yield the active 20S particle (V). Only some β subunits are specifically denoted. Other assembly factors and propeptides of non-catalytic β subunits are omitted for clarity.
B. Pba1–Pba2 complex maintains α rings competent for assembly. The Pba1–Pba2 complex can bind to select α subunits (I) and, via an unknown mechanism, participates in α ring assembly (II). The presence of Pba1–Pba2 bound to α rings ensures that they do not stray off-pathway into higher molecular weight complexes, such as α-ring dimers (II, upper panel), but continue on-pathway towards the 15S intermediate (III).
C. Pba3–Pba4 complex promotes the assembly of wild-type 20S proteasomes. In wild-type cells (top panel), the Pba3–Pba4 complex can bind strongly to either free α5 (I), or α5 in complex with other subunits (II). The Pba3–Pba4 complex acts as a scaffold (III) that stabilizes the interaction between α5 and α4, which allows α3 to enter (either alone, or in complex with α4 as shown). Subsequent binding events lead to the completion of the α ring (IV). The Pba3–Pba4 complex is then released and recycled during the transition to the 15S intermediate. In pba3Δ and/or pba4Δ cells (bottom panel), the absence of the Pba3–Pba4 scaffold (II) results in a second copy of α4 (either alone, or in complex with α4 as shown) competing with the normal addition of α3 (III). This leads to the formation of two types of α-ring (IV) which then become incorporated into 20S core particles, resulting in proteasomes of different subunit composition.